Reviewers: I'd suggest stepping through this one commit by commit

• Fixes Cannot subscribe shared folder if destination mailbox name is a prefix of source mailbox name #5146 by more strictly checking boundaries when converting between mboxlist dbnames and keys.
• Adds a new Cassandane suite "Shared" with tests for subscribing to shared mailboxes. I expect we'll accumulate other shared mailbox tests here over time.
• Bumps the subscriptions database version number from 2 to 3, and makes the upgrade-on-open path fix any broken v2 entries it finds. It fixes the bad subscriptions by string manipulation -- it does not check whether those mailboxes actually exist, nor whether the subscribed user has access to them. I don't think this is a problem, as I believe you can ordinarily be subscribed to mailboxes that don't exist, or which you cannot see, and you just won't see them, or they'll have the \Nonexistent flag if you LSUB or LIST (SUBSCRIBED) ... them.
• Renames the existing Cassandane "Lsub" test suite to "Subscriptions", and adds tests that create a v2 subscriptions db with good and bad records, and verify that it's correctly upgraded and fixed on first open.
• Extends the old and barely-used Cassandane::Mboxname class to better support non-admin namespace mailboxes, because I needed that functionality for these tests. These improvements should make this class useful in many other tests too.
• Also improves Cassandane::Util::Words a little, cause I needed lists of random words for the tests.