lib: table: use hash for exact-match lookups

lib: table: use hash for exact-match lookups

Most read accesses of route_table are actually exact matches where
walking down the tree is wildly inefficient.  Use a parallel hash
structure instead.

This significantly speeds up processes that are performance-bound by
table accesses, e.g. BGP withdraw processing.  In other locations, the
improvement is not seen as strongly, e.g. when filter processing is the
limiting factor.

[includes fix to ignore prefix host bits in hash comparison]

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>