actually handle timeout in mb_master:send_heartbeat
actually handle timeout in mb_master:send_heartbeat

Recent logs from larger cluster run have quite often mb_master crashes
because of this. And we're explicitly ignoring down nodes here.

We also lower timeout to reasonable 2 seconds. The idea is that in
theory we shouldn't block at all there, but in practice I'm seeing
enough cases where our send blocks (even given that we check for node
being officially up). And we simply don't care if we cannot send fast
enough because that likely means that communication channel between
nodes is clogged/down. Doing other work (including sending of next
heartbeats) is more important.

Change-Id: I7cee0de60607d410f61a1693716994f782b87b48
Reviewed-on: http://review.membase.org/6168
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>
Tested-by: Dale Harvey <491ccac09a36ad1e38b698f273b84fd8efc31071@arandomurl.com>
Reviewed-by: Dale Harvey <491ccac09a36ad1e38b698f273b84fd8efc31071@arandomurl.com>

membase/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,nimishzynga/ns_server,vzasade/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,daverigby/ns_server,pavel-blagodov/ns_server,membase/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,vzasade/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,nimishzynga/ns_server,pavel-blagodov/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,nimishzynga/ns_server,ceejatec/ns_server,mschoch/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,membase/ns_server,mschoch/ns_server,mschoch/ns_server,daverigby/ns_server,mschoch/ns_server,membase/ns_server,ceejatec/ns_server,vzasade/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server