interimap: avoid caching hierarchy delimiters forever in the database.

Following recommendation from
https://www.imapwiki.org/ClientImplementation/MailboxList#Hierarchy_separators

Instead, use null characters internally, and substitute them with the
local and remote hierarchy delimiters (which thus no longer need to
match) for IMAP commands.  This require a database schema upgrade to
alter the mailbox name column type from TEXT to BLOB.  We're using
SQLite's user_version PRAGMA to keep track of schema version; beware
that `.dump` doesn't export its value!

In logging messages, local and remote mailbox names are shown as is
(with their respective delimiters) while database mailbox names are
shown by replacing null characters with the *local* hierarchy delimiter.

Moreover for mailbox names specified on the command line or the
configuration file (with the "list-mailbox" option) the *local*
hierarchy delimiter should be used.

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