luci-base: implement session handling in luci.model.uci

Introduce luci.model.uci.set_session_id() and luci.model.uci.get_session_id()
to set and get the effective session ID respectively.

When a session ID is set, it is sent as `ubus_rpc_session` attribute to rpcd,
causing it to use per-session change directories, isolating LuCI changes from
the global system uci state.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>