Up from 1K.
Reduces the risk that someone could flush the list by tricking a user to
do many transfers to new hostnames.
Document the limit.
Follow-up to 03a792b186Closes#21200
Avoid never-ending growth.
When adding more entries, it now deletes the first entry in the list,
which is the oldest added entry still held in memory. I decided to avoid
a Least Recently Used concept as I suspect with a list with this many
entries most entries have not been used, and we don't save the timestamp
of recent use anyway.
The net effect might (no matter what) be that the removed entry might
feel a bit "random" in the eyes of the user.
Verify with test 1674
Ref #21183Closes#21190
