Added FIXMEs
Clean up: NativeTransaction becomes stealth, conversion traits deleted
Do not create db from read only transactions
Safer env access from Transaction

I'm not sure if that borrowing will be too restrictive on
a real-life project
Dropping DB from cache if it was deleted in transaction
Database is always non-owned now
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
Minor fixes
Marking NativeTransaction as NoCopy and NoSync