default backend to lmdb for image conversion and mean computation

lmdb is 10-15% faster than leveldb although it takes ~1.1x the storage.
This is usually irrelevant in prefetching since both are fast enough,
but more important lmdb allows multiple, concurrent reads for training
and evaluation several models on the same data.
switch examples to lmdb (except for custom data loaders)
Merge pull request #1128 from shelhamer/default-db-lmdb

default to lmdb for database storage