A database can be configured with a custom FilterPolicy object.
This object is responsible for creating a small filter from a set
of keys. These filters are stored in leveldb and are consulted
automatically by leveldb to decide whether or not to read some
information from disk. In many cases, a filter can cut down the
number of disk seeks form a handful to a single disk seek per
DB::Get() call.
Most people will want to use the builtin bloom filter support (see
NewBloomFilterPolicy() below).