MEDIUM: Add set agent pause|unpause unix socket command

The syntax of this new command is:

set agent <backend>/<server> pause|unpause

This command changes the behaviour of agent checks as follows:

In the case where an agent check is being run as a secondary check,
due to the agent-port parameter of a server directive, new checks
are only initialised when the agent is in the unpaused state. Thus,
setting the agent to pause will prevent any new agent checks from
begin initiated until the agent is set to unpaused.

In the case where an agent check is being run as the primary check,
due to either option lb-agent-check or http-check agent-hdr being set,
or for te processing of a secondary agent check that was initiated
while the agent was set as unpaused the processing of the agent check
when the agent is set to paused is as follows. All results that
would alter the weight, specifically "drain" or a weight returned by
the agent, will be ignored. The processing of agent check is otherwise
unchanged.

The motivation for this option is to allow weight setting 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>