Revision and WikiPage cleanup with IDBAccessObject interface.

Revision and WikiPage cleanup with IDBAccessObject interface.

* Replaced WikiPage::DATA_FROM_* constants with IDBAccessObject ones.
* Renamed IDBAccessObject constants a bit for visual consistency.
* Removed AVOID_MASTER parameter and replaced calling instances with READ_NORMAL.
  Instead of getting page_latest from the master and the revision from a
  slave, just get it all from the master in one RTT. Most callers used
  AVOID_MASTER (and now READ_NORMAL), so this case is barely hit anymore.

Change-Id: Ifbefdcd4490094b38e49bbb46c95fdb71b5c9e1a