11507: Move repack_writes() method from ArvadosFile to BufferBlock.
11507: Put self._pending_write_size update back under the lock.
11507: More small block packing fixes:
11507: Put bufferblocks into DELETED state when deleted so they don't getreused accidentally if there are dangling references. When packing smallblocks, update segments with the block hash instead of the bufferblock id toavoid dangling references.
11507: Test cases that reproduce bug
Merge branch '11510-sdk-extend-files' refs #11510
11510: Tweak interface to better match Pythons "io.IOBase" class.
11510: Fix truncate() to not change file pointer. Additional tests.
11510: Repack writes any time there's more than one segment referencing the same bufferblock.
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