Add header cache to DBObjectMap

OMap is used by PG to store PGLog, so it's called by each write operations.
Because leveldb don't play well in random read and "header_lock" limit the
concurrence. Add a LRU cache to avoid too much overload on seeking header.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Merge pull request #1582 from yuyuyu101/omap-cache

Add header cache to DBObjectMap

Reveiewed-by: Samuel Just <sam.just@inktank.com>