Live mastership migration support

Feature complete but I have a few more tests to perform (see [TODO](https://github.com/wajam/nrv/blob/manu-live-master-migration/nrv-core/src/test/scala/com/wajam/nrv/consistency/TestConsistencyMasterSlave.scala#L848-L850)s at the end of `TestConsistencyMasterSlave`). I would like to merge this pull request and hook it to the [CRTX](wajam/crtx) API before doing these tests.