services: improve Notary service locking

[AnnaShaleva]

Close #4077.

• [X] Allow parallel handling of notary requests with different main transaction.
• [X] Move expired request removal to PostBlock.
• [X] ~Allow parallel witness verification for requests with the same main transaction.~

@roman-khimov, please, check out the comments in code, the third issue is contraversial, I'm not sure how to safely do that.