hash: dynamically grow hash table

Dynamically grow the hash table index if the chains get too long.
If expansion doesn't help keep chain length short, then stop expanding,
to avoid bad behavior if there is a poor hash function.
Not a new idea, based on concepts in uthash.

Depends on my previous patch to restrict hash to power of 2.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
[profiling results: sum of cycles spent in hash_get/jhash with RIPE RIS
 test data (single simple BGP peer) improved to 69% of previously spent]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

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