[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.

vnodeWrite.c does not include dnode.h

move some functions in dnode into vnodeMain.c and vnodeWrite.c