Fix https://github.com/coreos/etcd/issues/5231.
The issue shows that slow CI can take more than 200ms
for purging. This increase the loop iteration to wait
up to 300ms in case the disk is being slow.
File lock interface was more verbose than it needed to be while
simultaneously making it difficult to support systems (e.g., Windows)
that only permit locked writes on a single fd holding the lock.
multiple cpu running may be slower than single cpu running, so it may
take longer time to remove files.
Increase from 5ms to 20ms to give it enough time.
