Improve database context manager robustness

[oberstet]

See crossbario/crossbar#2011

We should handle:

• LockError
• MDB_BAD_RSLOT

in

zlmdb/zlmdb/_database.py

Line 337 in 2ebd65a

def __enter__(self):

[oberstet]

adding a more or less clever microsleep and auto-retry: one problem is, the context manager isn't an asynch context manager, and hence any "microsleep" will need to run synchronously. we should be good with a micro (=very short) sleep though, and hence forcing sync should not bother the app much ..

[oberstet]

alright, this change should improve the situation c952f42

I'll leave this issue open for now .. as an anchor until we have more feedback in using the new code