Add configurable delay between purgeChangedPages batches

UDP flooding is possible when purging large numbers of files. This
change adds a command line switch to the purgeChangedPages.php
maintenance script to insert an artificial delay between purge batches.
The intent is to allow the network time to process the batch that was
just sent before flooding with a second batch.

`--sleep-per-batch` can be used in combination with `--batch-size` to
manage network and to some extent query load. A suggested starting point
would be to use `--sleep-per-batch=500` in concert with the default
batch size of 100 to target a maximum rate of 200 packets/second.

Bug: 55632
Change-Id: Ibfc54b1767f145098465404a2b23cd92852e41fd