MINOR: connections: Enhance tune.idle-pool.shared

MINOR: connections: Enhance tune.idle-pool.shared

There are two settings to control idle connection sharing across
threads.
tune.idle-pool.shared, that enables or disables it, and then
tune.takeover-other-tg-connections, which lets you or not get idle
connections from other thread groups.
Add a new keyword for tune.idle-pool.shared, "full", that lets you get
connections from other thread groups (equivalent to "full" keyword for
tune.takeover-other-tg-connections). The "on" keyword now will be
equivalent to the "restrict" one, which allowed getting connection from
other thread groups only when not doing it would result in a connection
failure (when reverse-http or when strict-macxonn are used).
tune.takeover-other-tg-connections will be deprecated.