Less mongo connections

[iLLiCiTiT]

New PR to release. Moved from PRhttps://github.com//pull/186

Changes

• Extracted basic MongoDB connection logic out of io in way to be reusable in different context than avalon's singleton and do not overcreate mongo connections

• created singleton based class AvalonMongoConnection for handling mongo connection

• to be able use different contexts was implemented AvalonMongoDB which holds Session attribute where avalon context is held

• when multiple objects of AvalonMongoDB are installed and one of them will uninstall the connection to mongo remains

  • connection to mongo is closed when all installed AvalonMongoDB change state to uninstalled

• this way we can reduce created connections per process

This depends on
pype	ynput/OpenPype#509
pype-setup	ynput/pype-setup#69