0.9.6.50: stability before creativity

0.9.6.50: stability before creativity
  * protect hash table operations with spinlocks on threaded builds =>
    ** no more memory corruption due to SHRINK-VECTOR in hash table code
    ** no more hangs caused by a corrupted NEXT-VECTOR that is cyclic
    ** as a side effect hash tables are thread safe (but it's not part of
       the contract, do your own locking)
    ** slower hash tables