resize() needs to limit the threads that may try to allocate newkvs

[rlei]

There may be multiple threads trying to call resize() and allocate their own new hash table (newkvs) before CAS it to the CHM._newkvs field, and only one will eventually succeed. Others allocation of the new (double sized) hash table will simply be a wasted effort.

The Java NBHM implementation limits the number of the threads that may try to allocate to 2. We should also port this.