Less mongo connections

[iLLiCiTiT]

Changes

• most changes described in avalon-core's PR where AvalonMongoConnection and AvalonMongoDB are implemented
• this gives possibility to create multiple objects holding differebt avalon contexts but won't create more connections to mongo

Description

• All avalon connections are reduced to one per process, because singleton mongo db connection is created in a way that can handle multiple contexts
• This does not mean much reduction in global space, since we use mongo also for logging (which is different connection), ftrack event server use it for storing events, users' ftrack action server is running in subprocess (have to create new connection). Same connections may be created also in hosts. With this PR is just reduced small part of an issue and got rid of io_nonsingleton at the same time.

🏴 this depends on ynput/avalon-core#186

[iLLiCiTiT]

PR moved to #509