Add new schema collection type and replace all usages of direct mongo collection for schema operations.

[nlutsenko]

This abstraction will give us an ability to use different syntax for Postgres, since schemas are not going to be represented as pure objects, but rather as tables and objects.

Also, cleans up the implementation and makes it more safe to build features on top.
Next stop - move schema api->mongo type conversions directly into MongoSchemaCollection.

[codecov-io]

Current coverage is 91.35%

Merging #943 into master will increase coverage by +0.05% as of 18a663a

@@            master    #943   diff @@
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  Files           72      73     +1
  Stmts         4301    4324    +23
  Branches       862     863     +1
  Methods          0       0       
======================================
+ Hit           3927    3950    +23
  Partial         10      10       
  Missed         364     364       

Review entire Coverage Diff as of 18a663a

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