Removed "page-lastedit" cache key trick from WikiPage
* Just rely on chronology protected and edit conflict handling.
The time a user spends looking at and editing pages is larger
than any normal slave lag anyway.
* However make sure that pages just made in the request are visible.
* In "master" datacenters, the slave lag will low anyway, and
callers make use of $flags when needed. In other datacenters,
the cache will itself be subject to lag anyway.
* Logging (DBPerformance log) shows this case is very rarely
hit anyway.
Change-Id: If34d67c02f9a7bf0a506ee8f3990697eb403a710