We weren't attaching resources to the row batch when starting a new
row group, so it was possible for string data to be overwritten. This
patch removes CloseStreams() and merges its functionality with
AttachCompletedResources() so it's not possible to destroy streams
without transferring the resources first. It also merges and removes
ScannerContext::Close().
Also adds test cases for IMPALA-720.
Change-Id: Ia8f40c7d39d8702716f1d337fe797e2696bd0fcb
