IMP-1329,IMPALA-924: Make ExchangeNode::Open() block until rows are available.

The bug: Coordinator::Wait() is supposed to block until rows become available for
consumption by the client. We rely on Wait() to determine when to advance the query
status to a 'ready' state and signal to the client that rows can be fetched.
Long fetch times can trigger client timeouts at various levels (socket, app, etc.).
Coordinator::Wait() simply opens the coordinator fragment's plan tree.
For most plan nodes, Open() does work to prepare the plan tree, s.t., GetNext()
returns quickly. However, for ExchangeNodes Open() used to not wait
until rows are obtained form the underlying stream receiver.
The fix: Make ExchangeNode::Open() block until rows are available.

Change-Id: I7b197eea11d21fd732414d96c899a17b2d99631c
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2128
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins