BUG/MEDIUM: stats: show servers state may show an empty or incomplete result

BUG/MEDIUM: stats: show servers state may show an empty or incomplete result

It was reported that the unix socket command "show servers state" returned an
empty response while "show servers state <backend>" worked.
In fact, both cases can reproduce the issue. It happens when the response can't
fit in one buffer.

The fix consists in processing the response in several steps, as it is done in
some others commands, by restarting where it was stopped after the buffer is
sent to the client.

This fix should be backported to haproxy 1.6