Replace leveldb with lmdb

One quite annoying thing of caffe is that the leveldb can only be accessed by one process at a time. For big database like imagenet, we need to duplicate the leveldb to run multiple models on different cards on one machine. lmdb is faster than leveldb [http://symas.com/mdb/microbench/], and most importantly, it supports multiprocess concurrency [http://symas.com/mdb/#features].

It is released under OpenLDAP license, which is a BSD-style license.