dump/restore: Use Xapian queries for batch-tag format

This switches the new batch-tag format away from using a home-grown
hex-encoding scheme for message IDs in the dump to simply using Xapian
queries with Xapian quoting syntax.

This has a variety of advantages beyond presenting a cleaner and more
consistent interface.  Foremost is that it will dramatically simplify
the quoting for batch tagging, which shares the same input format.
While the hex-encoding is no better or worse for the simple ID queries
used by dump/restore, it becomes onerous for general-purpose queries
used in batch tagging.  It also better handles strange cases like
"id:foo and bar", since this is no longer syntactically valid.