Fixes to masterPosWait() for master switchovers

Fixes to masterPosWait() for master switchovers

* Clean up return value types and docs.
* Handle master switch-over better w.r.t the job queue due
  to binlog name changes (the host portion). Previously the method
  would fail and trigger read-only mode when waiting on former
  master positions. Assume the the switch-over was done properly
  and thus return immediately.

Bug: T126436
Change-Id: Ib8c05a5c72d03a5c98e41b23c5653fc194b6d130