rdbms: expand on LoadBalancer ownership concept

rdbms: expand on LoadBalancer ownership concept

Enforce this pattern for the remaining LoadBalancer methods.
Carry this over into Database::close() to decide how loud the
error handling should be.

In LBFactory, clean up ownership of newMainLB()/newExternalLB().
The should have a null owner if called from outside the class
since the LBFactory does not track nor care about them anymore
in that case. Disable newMainLB() for LBFactorySingle as it
makes no sense and was broken.

Also remove some redundant abstract LBFactory methods that
just duplciate ILBFactory.

Bug: T231443
Bug: T217819
Depends-On: I7ed5c799320430c196a9a8e81af901999e2de7d0
Change-Id: I90b30a79754cfcc290277d302052e60df4fbc649