Better DB creation
Better DB creation

1. Using a fast child transaction, see comments in code why
3. empty name and null name are different for LMDB, so instead of bubbling
   up option use empty string as distinction

tempbottle/lmdb-rs,mneumann/lmdb-rs,krakatoa/lmdb-rs,frew/lmdb-rs,dropbox/lmdb-rs,krakatoa/lmdb-rs,vhbit/lmdb-rs,vhbit/lmdb-rs,frew/lmdb-rs,tempbottle/lmdb-rs,dropbox/lmdb-rs,mneumann/lmdb-rs