l3_db: refactor L3_NAT_DB_mixin

l3_db: refactor L3_NAT_DB_mixin

This patch refactors L3_NAT_DB_mixin to split out db operation and
rpc notification.

l3 plugin for routervm will implement the method for REST resource
operation as something like
    def op_resource():
        additional operation
        with session
            additional db operation
            db operation of super class => super().db_op_resoruce
            additional db operation
        additional operation
        l3 rpc notification

However, The current L3_NAT_DB_mixin intermixes db operations with l3 rpc.
So it is difficult to reuse the db operation code without l3 rpc.
This patch splits db operation from l3 rpc notification so that
db operation logic can be reused easily. Thus the l3 plugin for routervm
will be simplified with this patch.

Related to blueprint cisco-routing-service-vm
Related to blueprint l3-plugin-brocade-vyatta-vrouter

Change-Id: I88f6574f921596426e1a31c9ff2251aa6f4674b8