Use a pipe to wake up master_thread on mg_stop
Use a pipe to wake up master_thread on mg_stop

This fixes a 200ms delay in mg_stop, which got annoying when
running unit tests where each test started and stopped a server.

Change-Id: I102d356f8ca521448a149306fd6d1d2d2aac7872
Reviewed-on: http://gerrit.ent.cloudera.com:8080/336
Reviewed-by: Henry Robinson <3eca10f30041813f045165784e24b5a950a6cc7e@cloudera.com>
Tested-by: Henry Robinson <3eca10f30041813f045165784e24b5a950a6cc7e@cloudera.com>

brockn/squeasel,brockn/squeasel,danburkert/squeasel,danburkert/squeasel,brockn/squeasel,danburkert/squeasel,brockn/squeasel,danburkert/squeasel,brockn/squeasel,brockn/squeasel,danburkert/squeasel,danburkert/squeasel