libubus: add support for using channels

libubus: add support for using channels

A channel is a context that is directly connected to a peer instead of going
through ubusd. The use of this context is limited to calling ubus_invoke and
receiving requests not bound to any registered object.

The main use case for this is having a more stateful interaction between
processes. A service using channels can attach metadata to each individual
channel and keep track of its lifetime, which is not possible through
the regular subscribe/notify mechanism.
Using channels also improves request latency, since messages are passed
directly between processes.

Signed-off-by: Felix Fietkau <nbd@nbd.name>