In particular, we add a new FilterPolicy class. An instance of this class can be supplied in Options when opening a database. If supplied, the instance is used to generate summaries of keys (e.g., a bloom filter) which are placed in sstables. These summaries are consulted by DB::Get() so we can avoid reading sstable blocks that are guaranteed to not contain the key we are looking for.

This change provides one implementation of FilterPolicy based on bloom filters.

Other changes:

• Updated version number to 1.4.
• Some build tweaks.
• C binding for CompactRange.
• A few more benchmarks: deleteseq, deleterandom, readmissing, seekrandom.
• Minor .gitignore update.