Make DhcpRpcCallback a separate callback class

Make DhcpRpcCallback a separate callback class

RPC has a version of itself. In Neutron a plugin implements
several RPC interface, so a single RPC version doesn't work.
In Mixin callback class approach, RPC versioning depends on
each plugin implementation and it makes harder to maintain
RPC version appropriately. This patch series replaces mixin
RPC callback of server side with a separate class.

This commit handles server-side callback of dhcp-agent RPC interface.
DHCP-agent server-side callback class is moved from db/ to
api/rpc/handlers because it doesn't involve any db operations
and defining all RPC interfaces in a single place sounds reasonable.

Note that moving other DHCP-agent related RPC interface class
to api/rpc/handlers can be done in a separate patch as this patch
focuses on reorganizing the server-side RPC callback class.

Partial-Bug: #1359416
Change-Id: Ifb2a1bc0b7971995aae2856c9d4cd88c6dbc22d6