Document index upgrades

[ato]

Currently the upgrade process looks like:

1. Start OutbackCDX with desired --index-version option.
2. curl -XPOST http://localhost:8080/myindex/upgrade to start the upgrade.
3. The server will periodically log progress, note that the remaining time/records estimate is extremely inaccurate.

Upgrading may temporarily require double the disk space. If space is tight that maybe could worked around by periodic compacting at the expense of processing time.

If the upgrade is interrupted by a server restart it's safe to start the upgrade again. It'll scan through the records from the beginning and then continue where it left off.

Downgrading should also work as long as the index doesn't contain any records with extra JSON fields. Hasn't been test though.

The index should still be usable while it has a mix of record versions but there may be corner cases issues around deleting or replacing existing records. Inserting new records during the upgrade should be OK.

[kaij]

The upgrade worked absolutely flawless and fast on our test environment. Also CDXJ indexing worked after upgrade was done (for an existing record).