Keepalive future

Implemented 'keepalive' future. Associated options: keepalive, {connect_timeout, T}, {connect_limit, N}.
If 'keepalive' is on and amqp connection crushes:
	1. handler manager (amqp_handler_manager) stops his consumers (amqp_handler_consumer_sup)  and workers (amqp_handler_worker_sup).
	2. handler manager tries to connect to amqp server with timeout defined with option connect_timeout.
	3. If connect_limit exceeds the handler manager stops.
If 'keepalive' is off and amqp connection crushes the handler manager stops.