Add some connectivity diagnostics infrastructure

[pkgw]

This PR adds several pieces of diagnostics functionality. It might be helpful to read the commits in order, but the new user-facing features are:

• A new command librarian check-connections, intended to be run from a Librarian store host. This will look at all of the Librarian connections defined in ~/.hl_client.cfg and attempt to connect to each one. For each connection, it will ask for a list of all of its (available) stores and see if it can connect to them.
• The new Web UI endpoint /connectivity-check, accessible through the Admin menu, will run the above connectivity check on all stores attached to the Librarian server. This basically validates that all of the stores on this Librarian know how to talk to all of the stores on the other Librarians that this Librarian is aware of.

No user-facing documentation or tests; this is about as much time as I can spend on this stuff this week.

Edit: BTW, I tested this stuff by fooling around on folio2. Not exactly a torture test but I think it will all work?

[plaplant]

This is an awesome feature, thanks so much @pkgw! Testing on folio sounds good enough for me. If other issues pop up, we can always make a new PR at that point.