net: qrtr: Add forwarding support based on net id

Add support for QRTR to forward messages between network clusters. The
network subnet id's are attached to the QRTR subnodes in the MHI and
RPMSG/GLINK nodes.

The current forwarding decisions are done based on the net ids and are
wrapped in a single function for easier maintenance of forwarding
decisions later on. The function will return true once it determines
a subnet has no connection to another subnet.

The NEW_SERVER, DEL_SERVER, and DEL_CLIENT control messages should be
forwarded while the DATA and RESUME_TX commands should be passed along
to their destination node.

The nameservice is expected to send NEW_SERVER commands for it's entire
database instead of just the local service database with these changes.
This is to make sure new nodes get service notifications that came
earlier and were meant to be forwarded to the new node.

Change-Id: Ia6a1e952430f7d63bf361bc3f2427d492e982d96
Signed-off-by: Chris Lew <clew@codeaurora.org>