Fix race condition with sigalrm.

Summary: Took way too long to debug this... the issue was that if an
exception (like Server_initializing) got thrown before the alarm went
off, the alarm would not get unset. So at some point the signal handler
gets called and raises Server_busy.

Reviewed By: @jwatzman

Differential Revision: D1702848

Signature: t1:1702848:1416962236:1fc138cb08b254da69fb44912ac8149872929a5e