Decompose db_base_plugin_v2.py part 2

Decompose db_base_plugin_v2.py part 2

This patch contains method moving with minor modifications like updating
class names. IPAM specific methods were moved into ipam_backend_mixin and
ipam_non_pluggable_backend.

ipam_backend_mixin - contains code common for both backends (pluggable and
non-pluggable).

ipam_non_pluggable_backend - contains code specific for non-pluggable
IPAM implementation.

Partially-Implements: blueprint neutron-ipam

Change-Id: I6d68e4066ca472107def197c5a5afccbb6886a0e