MINOR: config: add a new tune.idle-pool.shared global setting.

MINOR: config: add a new tune.idle-pool.shared global setting.

Enables ('on') or disables ('off') sharing of idle connection pools between
threads for a same server. The default is to share them between threads in
order to minimize the number of persistent connections to a server, and to
optimize the connection reuse rate. But to help with debugging or when
suspecting a bug in HAProxy around connection reuse, it can be convenient to
forcefully disable this idle pool sharing between multiple threads, and force
this option to "off". The default is on.

This could have been nice to have during the idle connections debugging,
but it's not too late to add it!