Add low-level option to resync Connection to another database state with
preserving its live cache.

Wendelin.core needs this for its main ZODB connection because it needs
to carefully propagate ZODB invalidations into file invalidations and
semantically relies on objects staying in ZODB live cache for this to
work correctly:

https://lab.nexedi.com/kirr/wendelin.core/blob/455a54425c/wcfs/wcfs.go#L245