Partial work on Pseudo GTID:
- Config accepts:
  - TroubleshootModeEnabled (afecting Web interface to allow matching
slaves below a wider selection of instances)
  - PseudoGTIDPattern: a text/regexp pattern to look for in the binlog
events that indicate a unique entry (eg pseudo GTID)
API supports (incomplete):
- /api/match-below/:host/:port/:belowHost/:belowPort

Web interface:
- Recognizes a failed master state, and suggests a candidate slave (one
that is most advanced in replication
- Supports "safe mode"/"troubleshoot mode". Safe mode is the normal mode
where one refactors slaves via connectivity, stopping/starting, and
otherwise performing accurate calculations.
  Troubleshoot mode allows one to move slaves below other instances even
in conditions such as corruption of master, or moving a slave up up the
hierarchy to any ancestor.

instance_dao supports:
- looking up the latest pseudo GTID entry in an instance's binlogs
- Looking up a give nPseudo GTID entry in an instance's binlogs
instance_topology:
- (Incomplete) implementation for MatchBelow