WIP: Prototype Redis Sentinel implementation

Handles connect lookup. Tested connection in simple & working,
local setup. No automated tests. No fancy sentinel interation (failover
and friends). No documentation. No reconnects were tested (but should
work).

The Redis Sentinel client guidelines leave unspecified whether or not
the connection to the Redis Sentinel should be kept alive. This
implementation currently doesn't. I'm in two minds about this.

IDONTKNOW needs to be handled differently. If ANY Sentinel says
IDONTKNOW to a master resolution request, we need to bail.
Tentatively correct "Sentinel replying IDONTKNOW" handling
Implement other sentinel subcommands and sentinel autoupdate