* LMDB fix

LMDB was slow (>35min for ILSVRC2012_img_train.lmdb) when iterating
over the entries to find the keys. Now, it is possible to specify
the key_format, such that there is no overhead anymore.

Changes:
- use key_format to generate keys
- remove lmdb_open in reset-state (was open db twice)
- msgpack raised an exception, which is now catched

* use six to check stringtype

* rename key_format argument in lmdb and proper documentation

* small changes for prefetching

* some docs