Failover: support single configuration-string connect

Currently a simple connection can be connected via a single configuration-string, but failover groups require code setup; an API should support either simple or group connections using a suitable configuration string format.

Complications:

• the two APIs return concrete ConnectionMultiplexer vs abstract IConnectionGroup : IConnectionMultiplexer; we need to square this; options:
  • a new ConnectAny => IConnectionMultiplexer API
  • make ConnectionMultiplexer an abstract base-class with two concrete subclasses (huge change)
  • make ConnectionMultiplexer : IConnectionGroup in a trivial way, i.e. "a conspiracy of one" (this is probably our best option)
• token choice in the configuration string needs to consider that delimiters may exist in passwords etc; current thinking: ,|, between groups, which is syntactically unambiguous, but a little ugly