Add support for C bindings to the compaction V2 filter mechanism.

Test Plan: make c_test && ./c_test

Some fixes after merge.
Fix typo, add missing inclusion of state void* in invocation of
create_compaction_filter_v2_.
Merge pull request #224 from cockroachdb/spencerkimball/compaction-filter-v2-c-bindings

Add support for C bindings to the compaction V2 filter mechanism.