bgpd: rfapi: track outstanding rib and import timers, free mem at exit

bgpd: rfapi: track outstanding rib and import timers, free mem at exit

    While here, also make "VPN SAFI clear" test wait for clear result
    (tests/topotests/bgp_rfapi_basic_sanity{,_config2})

    Original RFAPI code relied on the frr timer system to remember
    various allocations that were supposed to be freed at future times
    rather than manage a parallel database. However, if bgpd is terminated
    before the times expire, those pending allocations are marked as
    memory leaks, even though they wouldn't be leaks under normal operation.

    This change adds some hash tables to track these outstanding
    allocations that are associated with pending timers, and uses
    those tables to free the allocations when bgpd exits.

Signed-off-by: G. Paul Ziemba <paulz@labn.net>