Acquire config.system.indexBuilds lock earlier and with collection lock

We push down "config.system.indexBuilds" lock acquisition to a low-level helper. This means that in the index build abort critical section, if our operation has been interrupted, the lock acquisition to remove the index build entry can fail.

The current solution to prevent a crash is an UninterruptibleLockGuard, which is not ideal. Though we will not try to take an X lock, which could be problematic, we should just make the caller responsible for lock acquisition.