bootstraping on exiting multi source replication with channel

[svaroqui]

How to reproduce

Bootstrap a named source replication , switchover and rebootstrap
Existing replication stream on channel is not reset so the topology is changed to loop
Manual workaround
reset slaves all for channel "bench" in bootstrap code

When replication is re bootstrap it would be best to keep the last seen master instead of re start on fist node in the list as master