Detect missing heartbeat ACKs and broken connections faster

Waiting twice the heartbeat interval since the last ACK can make us miss dead connections for more than 10min in which no push messages get received.

This change requires the ACK to come within 90 seconds of sending the heartbeat ping, otherwise the connection is considered dead which will trigger a re-connect.

We use a high timeout of 90 seconds, because in some mobile networks (like in trains) ping times of more than 60 seconds have been observed in practice.

Change-Id: I8e63843400b47e1a6d23faf180f9446bcb5e0cd8