Clients that don't Fetch can cause Impala to busy spin in ScannerThreads

The scanner thread registers with the token mgr in Prepare() (Execute from the client) and doesn't issue work to the io_mgr until GetNext() (Fetch from the client). After registering with the token mgr, the token mgr will offer tokens (HdfsScanNode::ThreadAvailableCb) which spins up scanner threads. Since there are no ranges issued, the scanner threads busy spin keeping a core busy per thread.

This is bad anytime there is a large gap between Prepare/Open() and GetNext(). This can happen because the client went away (spinning until query timeout) or other parts of the plan take a long time (i.e. processing the build side of a join).