MEDIUM: Add enable and disable agent unix socket commands

MEDIUM: Add enable and disable agent unix socket commands

The syntax of this new commands are:

enable agent <backend>/<server>
disable agent <backend>/<server>

These commands allow temporarily stopping and subsequently
re-starting an auxiliary agent check. The effect of this is as follows:

New checks are only initialised when the agent is in the enabled. Thus,
disable agent will prevent any new agent checks from begin initiated until
the agent re-enabled using enable agent.

When an agent is disabled the processing of an auxiliary agent check that
was initiated while the agent was set as enabled is as follows: All
results that would alter the weight, specifically "drain" or a weight
returned by the agent, are ignored. The processing of agent check is
otherwise unchanged.

The motivation for this feature is to allow the weight changing effects
of the agent checks to be paused to allow the weight of a server to be
configured using set weight without being overridden by the agent.

Signed-off-by: Simon Horman <horms@verge.net.au>