Implement concurrent facilities for Store

* Introduce ConcurrentUnorderedDictionary, a concurrent insertion safe
  dictionary.

* Introduce ConcurrentFixedLenghtVector, a storage structure that may be
  grown safely by multiple threads

* Add AbstractTable::copy_structure with callbacks for backing
  structures, allowing Store to use concurrent structures

* Add AbstractFixedLengthVector so that ConcurrentUnorderedDictionary
  may safely fill in where FixedLengthVector was used