Performance optimization for bloom filters.

This reduces a peer's ability to attack network resources by
 using a full bloom filter, but without reducing the usability
 of bloom filters.  It sets a default match everything filter
 for peers and it generalizes a prior optimization to
 cover more cases.
Add MurmurHash3() unit test

Useful as a source of test vectors to anyone re-implementing bloom
filters.
Squashed 'src/leveldb/' changes from a02ddf9..be1b0ff

be1b0ff On Mac OS X fsync does not guarantee write to disk. Use fcntl F_FULLFSYNC instead.

git-subtree-dir: src/leveldb
git-subtree-split: be1b0ff1fcd6ad820a7fd111ac671fb51cc68001
Merge leveldb subtree change.

* On Mac OS X fsync does not guarantee write to disk. Use fcntl F_FULLFSYNC instead.
Merge pull request #2916 from sipa/leveldb-osxfsync

Import OSX fsync change from LevelDB subtree
Merge pull request #2914 from gmaxwell/bloom_faster

Performance optimization for bloom filters.
Merge pull request #2915 from petertodd/murmurhash-tests

Add MurmurHash3() unit test