Only reset session on error in process, override transcode with pledge

[mkitti]

This is another approach to making transcode save the frame content size.

Previously via #4, the compression session was reset via startproc. The
only reason I can glean to reset the session would be if process errored.

Now startproc only resets the buffers, but does not reset the stream.
The compression context is only reset if an error occurs during process.

After these changes we can now call ZSTD_CCtx_setPledgedSrcSize from transcode
in order to encode the decompressed size in the frame header.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 35.56%. Comparing base (1915ffb) to head (874e49c).

Files	Patch %	Lines
src/compression.jl	75.00%	2 Missing ⚠️
src/libzstd.jl	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
- Coverage   36.96%   35.56%   -1.41%     
==========================================
  Files           5        5              
  Lines         560      568       +8     
==========================================
- Hits          207      202       -5     
- Misses        353      366      +13     

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[nhz2]

Good, the new test_roundtrip_seekstart is catching the bug in this PR.

It is best practice for startproc to always fully reset the state of the codec.

[mkitti]

We should figure out how to pledge the size from a transcode call to the normal ZstdCompressor since we know the entire buffer size. Perhaps we should add a pledged size field to ZstdCompressor to enable this so that we can pledge the size in startproc afte we reset the stream.