[FUSE] basic cow implementation & rudimentary tests

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+# Cowtest
+
+### Table of Contents
+1. [Introduction](#introduction)
+2. [Usage](#usage)
+3. [Tests](#tests)
+
+
+
+# Introduction
+
+This test suit is for verifying that the fuse cow implementation works correctly. It can verify that read and writes on the cow fuse client work correctly and that the cow server merges the image correctly.
+
+
+# Usage
+
+### Parameters
+- `-c <path>` generates a test image at given path. This image should be loaded for the tests to work.
+- `-t <path>` runs the tests on the image at the given past.
+- `-v <path>` verifies that previous tests on the image were successful (also reads the image completely).
+- `-r <mountedImagePath> <normalImagePath>` random writes and changes size of two images. After a key press both images are compared for equalness. 
+- `-x <mergedImagePath> <normalImagePath>` Checks both images are compared for equalness. 
+### Example usage for standard test
+
+1. Generate the test image with `-c <path>` and copy it to the image location of the dnbd3 server. Also make sure that the cow servers `OriginalImageDirectory` points to the same Directory or copied in that Directory too. This step is only needed once for  setting up.
+2. Start the dnbd3 and cow server.
+3. Mount the image in cow mode.
+4. Run the test with `-t <path>`, where the path points to the mounted image.
+5. Optional verify again with `-v <path>`.
+6. Optional unmount the image and then load it again (with `-L <path>` in the fuse client). Then verify the loaded image with `-v <path>`.
+7. Unmount and merge the image.
+8. Verify the merged image from the cow server with `-v <path>`.
+
+### Example usage for random writes
+1. Generate the test image with `-c <path>` and copy it to the image location of the dnbd3 server. Also make sure that the cow servers `OriginalImageDirectory` points to the same Directory or copied in that Directory too. This step is only needed once for  setting up.
+2. Copy the generated image to another location.
+3. Start the dnbd3 and cow server.
+4. Mount the image in cow mode.
+5. Run the test with `-t <mountedImagePath> <normalImagePath>`, where the `<mountedImagePath>` points to the mounted image and `<normalImagePath>` points to the copied image on the disk.
+6. After some time press enter and both images will be compared for equalness.
+7. Unmount the image and merge.
+8. Run `-x <mergedImagePath> <normalImagePath>` where the `<mergedImagePath>` points to the merged image and `<normalImagePath>` points to the copied image on the disk. This will verify that the merged image is equal to the image on the disk.
+
+
+
+# Tests
+
+### TestSingleBit
+Reads the first block and verifies that all bits are 0. Then it sets the first bit to 1 and writes it.
+This test, tests basic functionality and verifies that the image is still 'clean'.
+Then it sets a single Bit in the second block to 1, to verify padding works correctly.
+
+| offset | size | 
+| -------| -----| 
+| 0 | 2 * DNBD3_BLOCK_SIZE|
+
+
+### WriteOverTwoBlocks
+Tests that continuous writes over two DNBD3_BLOCK's are possible.
+
+| offset | size | 
+| -------| -----| 
+| DNBD3_BLOCK_SIZE * 3| size: DNBD3_BLOCK_SIZE * 2|
+
+
+### WriteNotOnBlockBorder
+Verifies that writes not aligned to block borders (multiples of 4096).
+
+| offset | size | 
+| -------| -----| 
+| DNBD3_BLOCK_SIZE * 11 - DNBD3_BLOCK_SIZE / 2| DNBD3_BLOCK_SIZE * 2 |
+
+
+### InterleavedTest
+
+| offset | size | 
+| -------| -----| 
+|DNBD3_BLOCK_SIZE * 35 | DNBD3_BLOCK_SIZE * 10|
+
+### WriteOverL2
+Tests that continuous writes over L2 borders are possible.
+
+| offset | size | 
+| -------| -----| 
+|l2Capacity * 2 - DNBD3_BLOCK_SIZE | DNBD3_BLOCK_SIZE * 2 |
+
+
+### MultipleWrites
+Writes multiple times on the same Blocks different data. The separate writes can be delayed with the `-d` parameter. This is useful to test if uploading the same blocks multiple times works as intended.
+
+| offset | size | 
+| -------| -----| 
+| 100 * DNBD3_BLOCK_SIZE * bitfieldByteSize | DNBD3_BLOCK_SIZE * 10 * bitfieldByteSize |
+
+
+### fileSizeChanges
+Tests file size changes. First in increases the file size with a truncate by 2 * l2Capacity. It then checks that all the bits in the new allocated space are set to 0. Then it writes data to it to verify writes are possible. After that it truncates it back to the original size. Then it truncates it back to
+the original size + 2 * l2Capacity and verifies that the again all bits in the new allocated space are 0 (so that the before written data is set to 0 again).
+
+### LongNonAlignedPattern
+This test writes writes an long pattern over 3 l2 borders. The pattern repeats chars from 0 to 254, so it's not a multiple of 4096, which therefore results that all Blocks are filled with different data. Also, this test is not block aligned.
+
+| offset | size | 
+| -------| -----| 
+|l2Capacity * 3 - 1|l2Capacity + 2|