The `monitor-delay-interval` has been added to the DB config so that
users can change the time period between WAL checks after a file
change notification has occurred. This can be useful to batch up
changes in larger files in the shadow WAL or to reduce or eliminate
the delay in propagating changes during read replication.
Setting the interval to zero or less will disable it.
This commit refactors out the complexity of downloading ordered WAL
files in parallel to a type called `WALDownloader`. This makes it
easier to test the restore separately from the download.
