Global re-index

[ryanb]

There should be a convenient way to trigger a global re-index for times when the xapit index definition changes. We already have the rake xapit:index task, but that cannot be called alongside the syncing process because of the lock on the writable database.

Perhaps a special XapitChange record could be created which tells the separate process to re-index everything. It would be nice if this re-indexing worked on a separate database file and then swapped it out when it was done.

Now what about XapitChange records which get created during the global re-indexing? These may or may not need to take effect depending on the current progress.