The lock() calls which were in place only prevent concurrent calls from
different threads; however, the critical section could still be reached
from two async calls in the same thread a la:

backend.prepare.begin ((s,r) => {...});
backend.prepare.begin ((s,r) => {...});

The bodies of the handlers would not literally execute concurrently, but
would be interleaved if each were sufficiently long. Our fix simply
ignores any calls which happen while one is still working.

(yield backend.prepare(); yield backend.prepare(); was already safe, of
course)

Helps: bgo#652637 - Don't hold locks across async calls