tryLock is not released properly

[jkawamoto]

We still get thousand of goroutines that waits for locking a tryLock:

goroutine 458763 [chan receive, 94 minutes]:
lukechampine.com/us/renter/renterutil.(*tryLock).Lock(...)

It looks like https://github.com/lukechampine/us/pull/167 isn't enough and there might be another miss of releasing a lock.