Rewrite connection module

[insspb]

I had a time to look at connection module, that originally was made in 2016-2017. In nearest past there was a lot of question and bugs related to connection.
By my opinion our old connection module did a lot of work, that should not be done in this package at all.
For example there is some things that by my opinion is not correct:

• Setting of some connection defaults - we should not do it, as same defaults will be set in upstream mongoengine/pymongo packages. We do not do anything with these defaults, but do the work that hard to track. Also as it is hard to track changes in parent packages, that should be implemented here.
• Replacing mongomock urls - in this package mongomock support was dropped in version 0.8.1, but mongomock settings handled by mongoengine upstream package. We should not try to do the work, that is done in package with much more contributtors.
• Different behavior on dict or list settings - old implementation had two types in return of connection functions. In case one db used it was dict, in case of multiple dbs it was list. It is not correct and can be considered as unexpected behavior.

So at the end this Pull request will:

• Removed unexpected set of defaults.
  User is now fully responsible to set correct defaults, or defaults from upstream mongoengine and pymongo will be used.
• Connection property of flask-mongoengine MongoEngine class now will always return dict with one or many database connections.
• Connection handling now the same for one connection or many connections. Before this PR it was difference when 1 or many DB used.
• Updated tests for new behavior.

TODO before merge:

• Documentation update

Should fix:

• MONGODB_ prefix not working #385
• AUTH_SOURCE Not supported #381
• Can not connect to mongodb on cloud server #386
• Compatibility issue with mongoengine 0.24.1 #451
• Tests fail when mongomock is installed #420

I honestly hope for any comments, and for some test users, as it is deep flask-mongoengine behavior change. Code reviews are welcomed on any stage.