bighash: add optional automatic bucket growth

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 change when I tried running a similar benchmark with bighash.

The hairy bighash_grow function is copied from the IVS TCAM module.
BigHash: fix autogrow flag
Merge into master from pull request #24:
bighash: add optional automatic bucket growth (https://github.com/floodlight/bigcode/pull/24)