[2.8.1 Backport] CBG-1222 - Pre-2.8 cbgt index definitions not being removed post-upgrade

In 2.8, Sync Gateway changed the naming convention for cbgt indexes to avoid long DCP names (CBG-626).  It looks like this can result in two index definitions existing per database post-upgrade, causing duplicate import DCP feeds to be started by cbgt.

Needs investigation to confirm the behaviour, and identification of a way to remove pre-2.7 index definitions on upgrade.  Including in post-upgrade is available as a last resort, but would be preferable to seamlessly upgrade, if that's possible without breaking rolling upgrade.