sstable/block: additional refactoring

[jbowens]

sstable/block: move Checksummer, ChecksumType

Move sstable.ChecksumType and sstable.Checksummer types into the sstable/block package.

sstable/block: add Trailer type

Add a fixed-width block.Trailer type, and use it to avoid subtle usage of blockBuf.tmp to pass encoded trailers.

sstable/block: clean up cache.Value and BufferPool union

Clean up the block.CacheValueOrBuf union type, renaming it to block.Value and adding a single block.Alloc entry point for constructing values.

[cockroach-teamcity]

This change is