Fix resource exhaustion with upstart.

When high VM churn is encountered dhcp-all-interfaces got tangled up
and leaked instances until upstart run out of filedescriptors.

This happened because we were locking on every invocation, and the
lock logic was disabling the single-interface code - so we always
probed every interface every time - and this was serialized across
every instance of dhcp-all-interfaces.

Change-Id: I4ef1fb83d978cc99d4cb9cc2666bde302c6db7d1