Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/15098
Compute the base block checksum in `CompressAndVerifyBlock()` before `verify_compression` decompresses the compressed output. Parallel compression stores that base checksum on the block rep so worker threads can keep the CPU-heavy checksum and verification work off the ordered write path; the writer only applies the offset context modifier before writing the trailer.
Update the regression test to cover both serial and parallel compression paths.
Reviewed By: pdillinger
Differential Revision: D115950785
fbshipit-source-id: b5e47f7d7c41680e7e7907443fc805bc0d043d6e
(cherry picked from commit 92a6308300)
8941ff459d