bighash: add optional automatic bucket growth

Reviewer: @jnealtowns

This is enable by passing BIGHASH_AUTOGROW to create or init. I saw a nice
performance improvement in the TCAM module benchmarks when I added the auto
resize feature there, probably due to better cache locality. I didn't see any
performance improvement when I tried running a similar benchmark with bighash.
But I didn't see a performance regression either, and this will prevent the hashtable from
degrading to a linked list if we size it incorrectly.

The hairy bighash_grow function is copied from the IVS TCAM module.