hydra-queue-runner: Start as many builds as possible on each iteration

Because we don't start a build if a dependency is already building,
it's possible that some or all of the $extraAllowed highest-priority
builds in the queue are not eligible.  E.g. with $extraAllowed = 32,
we might start only 3 builds even though there are thousands in the
queue.  The fix is to try all queued builds until $extraAllowed have
been started.

Issue #99.
Speed up findBuildDependencyInQueue

This was taking a long time due to the giant SQL query.

Issue #99.
hydra-queue-runner: Don't unlock builds we just started

This happens if the previous iteration took more than 60 seconds.
Then the queue runner may think that builds failed to start properly
and unlock them, e.g.

build 5264936 pid 19248 died, unlocking
build 5264951 pid 19248 died, unlocking
build 5257073 pid 19248 died, unlocking
...