bgpd: improve labelpool performance at scale

bgpd: improve labelpool performance at scale

    - double the size of each new chunk request from zebra
    - use bitfields to track label allocations in a chunk
    - When allocating:
        - skip chunks with no free labels
        - search biggest chunks first
        - start search in chunk where last search ended
    - Improve API documentation in comments (bgp_lp_get() and callback)
    - Tweak formatting of "show bgp labelpool chunks"
    - Add test features (compiled conditionally on BGP_LABELPOOL_ENABLE_TESTS)

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