Improve lag/flag behavior and docs in LoadBalancer

Improve lag/flag behavior and docs in LoadBalancer

* Set DBO_DEFAULT if "flags" is not set. MediaWiki config bootstrapping
  does this, but for outside users, it should still be the default too.
* Make "is static" do more than no-oping DatabaseMysqlBase::masterPosWait().
  It now also treats servers with this flag as having 0 lag at all times.
  This makes it far more useful, as the point of the flag is to mark DBs
  that are archival clones of each other and may not even replicate.
* Add more ILoadBalancer usage documentation.

Change-Id: Ib0b44e42e9f4c57a292d6a6ea17beb454db464bc