Spun off from T114271.

We need scripts and processes to do a planned switch from master datacenter A to B:

a) Go read-only on the app level (mostly MediaWiki)
b) Make sure write traffic stops
c) Go read-only for all data stores
d) Wait for all data stores in the B datacenter to catch up and be in sync with A
e) Make the B datacenter the new master datacenter (systems and app level)
f) End read-only mode

Read-only mode should be as short as possible so we can actually test this.