run poor man's alerts checks in separate child process
run poor man's alerts checks in separate child process

Under heavy load even disk_sup access may be very slow. So in order to
make menelaus_web_alerts_srv always responsive, I separated
potentially expensive alerts checking logic into it's own process.

This should fix numerous 500 errors from pool details caused by
timeouts accessing alerts server.

Supervisor normally uses shutdown exit on it's childs, which will
ensure that child process gets terminated because neither alerts
server, nor checker process trap exit.

Change-Id: I332683cdcfe76975b899043a8444874ceb5b8285
Reviewed-on: http://review.membase.org/6446
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Steve Yen <ef850f73f7832052d3c6d9b5ddd3a1716d9a57f1@gmail.com>

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