pThreadObj is freed when thread exits

[TD-445]

Only increment wWorkerPool.nextId when wWorkerPool.nextId is launched

This patch adds more error handling code:
1. In dnodeAllocateWqueue(), if the allocation of pWorker fails, any
allocated memory should be unwound and returns a NULL queue. This is
a better function contract where either all allocation succeeds or none
succeeds.
2. In vnodeOpen(), handle allocation failure for pVnode.