The create function took |num_keys| as an int, but callers and implementers wanted it to function as a size_t (e.g. passing std::vector::size() in, passing it to vector constructors as a size arg, indexing containers by it, etc.).  This resulted in implicit conversions between the two types as well as warnings (found with Chromium's external copy of these sources, built with MSVC) about signed vs. unsigned comparisons.

The leveldb sources were already widely using size_t elsewhere, e.g. for key and filter lengths, so using size_t here is not inconsistent with the existing code.  However, it does change the public C API.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=101074871

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.