hash: force size to be a power of 2

hash: force size to be a power of 2

By forcing the hash table size to be a power of 2, a potentially
expensive divide can be replaced by a mask operation. Almost all
usage of the hash table was using default size of 1024. Only places
with different size was thread library (1011) and bgp aspath.

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