Various database class cleanups

Various database class cleanups

* Refactor out some code duplication in query() into a
  separate private method.
* Remove the total master/slave query profiling, which is not
  necessary and redundant.
* Provide a default implementation for reconnect().
* Make reconnect() catch errors so it can match the docs that say
  it returns true/false to indicate failure. Likewise for ping().
* Optimize ping() to no-op if there was obvious recent activity.
* Move the ping() round in JobRunner to approveMasterChanges.
  This way, all commit rounds benefit from this logic.
* Add more doc comments for DatabaseBase fields.

Change-Id: Ic90ce2be4187244a0e8d44854c39d4b78be8e642