Change ut-compressed_stream backwards test a bit.
Add ut-compressed_stream backwards_fs. Just the backwards test using a file_stream.
Get rid of docstrings mentioning nothrow exception guarantee.
Check for non-appending write before doing perform_seek.
Don't increment streamBlocks too soon.
Fix race conditions when reading back.

Introduce a buffer state so that each buffer knows if it has the right
data or not.
Add compressed_speed_test.