Connection recovery

Client (Subscription) stacks requests while server is unreachable. After connection has been reestablished, the client (Subscription) checks which requests were sent successfully and then resends the rest. After this procedure normal sync is resumed.

- commented out line 587 in MongoProvider (throw new Exception('not correctly implemented');)
- modified method sendRequest in Subscription (may need refactoring)
- this branch is based on commit adc09f28e99d20b5bf206c90983e0dda55ad61c6 (19.02.2014, 13:09 by hleumas)
- http://devopsreactions.tumblr.com/post/57507135731/trying-to-push-a-merge-conflict-fix-and-receiving-a