Removed "page-lastedit" cache key trick from WikiPage

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