Summary:
Although RocksDB falls over in various other ways with KVs
around 4GB or more, this change fixes how XXH32 and XXH64 were being
called by the block checksum code to support >= 4GB in case that should
ever happen, or the code copied for other uses.

This change is not a schema compatibility issue because the checksum
verification code would checksum the first (block_size + 1) mod 2^32
bytes while the checksum construction code would checksum the first
block_size mod 2^32 plus the compression type byte, meaning the
XXH32/64 checksums for >=4GB block would not match about 255/256 times.

While touching this code, I refactored to consolidate redundant
implementations, improving diagnostics and performance tracking in some
cases. Also used less confusing language in those diagnostics.

Makes https://github.com/facebook/rocksdb/issues/6875 obsolete.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6978

Test Plan:
I was able to write a test for this using an SST file writer
and VerifyChecksum in a reader. The test fails before the fix, though
I'm leaving the test disabled because I don't think it's worth the
expense of running regularly.

Reviewed By: gg814

Differential Revision: D22143260

Pulled By: pdillinger

fbshipit-source-id: 982993d16134e8c50bea2269047f901c1783726e