bloom: reworked bloom filter

Added clean public API:
 * eblob_bloom_get()
 * eblob_bloom_set()

Replaced per-index block bloom with per-bctl bloom. There is no need to
traverse index blocks tree if there is no key in bctl.

This also reduces pressure on memory allocator, memory fragmentation and
reduces eblob memory consumption.

Now bloom filter works for non-hashed values.
We using both fnv1a and djb2a internally, but because all internals now
hidden behind API this may be easily changed in future releases.