Fix MongoDb operations to use transaction on both read and write

[Arkatufus]

Fixes #346 Fixes #345

Changes

• Journal
  • Remove subscription to new events, tags, etc
  • All operations are covered using a single cancellation token
  • Transactions are applied to the whole operation
  • Extract queries into its own static class
• Snapshot store
  • All operations are covered using a single cancellation token
  • Transactions are applied to the whole operation

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• [x] This change follows the Akka.NET API Compatibility Guidelines.
• [ ] I have reviewed my own pull request.

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[Aaronontheweb]

@Arkatufus FYI, tests are not running for the majority of the suite

[Arkatufus]

it should be fixed now

[Aaronontheweb]

Akka.Persistence.MongoDb.Tests.MongoDbTransactionCurrentAllEventsSpec.ReadJournal_query_CurrentAllEvents_should_find_existing_events Failed: Timeout 00:00:30 while waiting for a message of type System.String

Gonna see if that fails again upon a re-run

[Arkatufus]

Note for future developers:

As of this PR, the session object used inside a transaction is not thread safe. Never run 2 or more concurrent DB operations inside a transaction, they will cause the transaction to fail horribly. This is easily spotted by looking for any detached Task (any Task that is not await-ed), or the use of Task.WhenAll or Task.WhenAny. Just. Don't.