consolidate detach-replica and detach-replica-master-host

[shlomi-noach]

Fixes #779

This PR consolidates detach-replica and detach-replica-master-host. It makes detach-replica behave exactly like detach-replica-master-host. This makes detach-replica safe to work with GTIDs. See #779 for more details.

[ggunson]

What specifically does detach-replica-master-host do that detach-replica doesn't?

[shlomi-noach]

detach-replica-master-host changes the master hostname from some.host.name.com to //some.host.name.com.

detach-replica (before this PR) changes Master_log_file from mysql-bin.0001234 to //mysql-bin.0001234:29043857. This worked well for non-GTID setups, but for AUTO_POSITION=1 it would error.

The advantage of the detach-replica approach is that the replica still points to the same master, just unable to replicate.