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.

When set pause is in effect the processing of an agent check run as the
primary check, due to either option lb-agent-check or http-check
agent-hdr being set, or processing of a secondary agent check that was
initiated while the agent was set as unpaused 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 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.

The default state is unpaused.

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

diff --git a/doc/configuration.txt b/doc/configuration.txt
index 3a63af4..e5d07ea 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -11515,6 +11515,30 @@ prompt
 quit
   Close the connection when in interactive mode.