search: refactor DatabaseSearch to take a load balancer instance

search: refactor DatabaseSearch to take a load balancer instance

Also make the update() methods of the subclasses use DB_MASTER as they
should. This avoids read-only errors.

In addition, avoid passing a dummy argument of null in some cases
within SearchEngineFactory::create(). Fix some dynamic calls to
static methods too.

Change-Id: Id94f34994b0f9c18e23ef30cb2fe895e6dedd09c