REST API/RTC: Fix divergence when two offline users reconnect.

Prevent Real-Time Collaboration sessions from becoming out of sync when an offline editor returns to an online state. The change ensures that the updates are merged rather than discarded when each client is at a different cursor after reconnecting or experiencing slow network conditions.

Props alecgeatches, peterwilsoncc, maxschmeling, joefusco.
See #64622.


git-svn-id: https://develop.svn.wordpress.org/trunk@62298 602fd350-edb4-49c9-b593-d223f7449a82