pimd: simplify pim upstream state transitions

pimd: simplify pim upstream state transitions

This is another follow-up change to the reg-state and up-join-state
separation. The upstream join state machine can now respond to
JoinDesired macro changes independent of router role.

I have also dropped the PRUNE state from the upstream-join-state
enumeration. RFC4601 only defines JOINED and NOTJOINED states. And PRUNE
can really be replace by NOTJOINED.

Signed-off-by: Anuradha Karuppiah <anuradhak@cumulusnetworks.com>
Reviewed-by: Donald Sharp <sharpd@cumulusnetworks.com>
Ticket: CM-14700
Testing Done: Register state machine in FHR only, combined FHR-RP and
FHR-RP-LHR/all-in-one setups. Also ran pim-smoke.