rdbms: allow construction of Database objects without connecting

rdbms: allow construction of Database objects without connecting

* Database::factory() supports a $connect parameter, that defaults
  to NEW_CONNECTED (current behavior) but can also be NEW_UNCONNECTED.
* Add tests asserting the type of various instances returned from
  Database::factory().
* Clean up sqlite "conn" field handling to handle cases of it
  not being set, just as other classes do.
* Add some comments about the return type of doQuery().

Change-Id: Ic0837cfdb35326c2045133d664abd29043d48c03