default to lmdb for database storage

lmdb is faster at read and supports concurrent access for training and evaluating multiple networks on the same data at the same time.
As a further step I vote to switch the caffe.proto default backend to lmdb from leveldb in dev with a note in the release since this changes existing model defaults. Thoughts?

Certain examples like cifar and mnist siamese have their own custom data tools that are left as leveldb.