Rename replication subscription to replication session

Also renamed subscribe/unsubscribe to open session/close session and defined new replication paths. The old  replication paths are deprecated but still temporarily supported:

Open replication session: ```POST /replication/master/:token/sessions```
Close replication session: ```DELETE /replication/master/:token/sessions/:session_id```
Replicate transaction: ```PUT /replication/slave/sessions/:session_id```