stats: separate stats arrays per writer

This reduces contention for commonly updated cachelines. Reading stats will be
slower because it has to aggregate from all the writers, but we expect far more
writes to stats than reads.
stats: allocate shared memory for stats

This allows forked processes to write to stats and have their parent see the
updates.
Merge into master from pull request #215:
Shared memory, per-thread stats (https://github.com/floodlight/ivs/pull/215)