CDRIVER-1571 slow initiator cancels topology scan

Async ismaster commands started before a slow call to a blocking stream
initiator had the initiator's duration subtracted from their timeouts.
The initiator and the async commands use the same initial timeout value,
so once an initiator times out, all async commands started beforehand
now have 0 seconds remaining. They're canceled before they run.

This change relies on the overall timeout applied to mongoc_async_run
instead of per-command timeouts.