book and ECO mapping updates
Fix possible race condition, in which thread pool is resized then
a "new" command is executed, which clears the hash table. The new
threads may not have been started, in which case the call to clear
the tables will crash.

This fix just omits the operation if the thread is not initialized,
which in this case is ok, but a more general fix would provide a
mechanism to wait for all threads to start after a resize.