Protect WAN cache sets() against uncommitted data

Protect WAN cache sets() against uncommitted data

This generally only effects wikis with no slave DBs,
but also matters if the master has non-zero LB load.
If the master ends up being used for DB_SLAVE, care
should be shown for cache-aside writes

Interesting WAN cache events are now logged.

Change-Id: I2cd8e84138263c13ea23beb9ab3d7562340e1fd3