Adjust nodeBitmapIndexscan.c to not keep the index open across calls,

Adjust nodeBitmapIndexscan.c to not keep the index open across calls,
but just to open and close it during MultiExecBitmapIndexScan.  This
avoids acquiring duplicate resources (eg, multiple locks on the same
relation) in a tree with many bitmap scans.  Also, don't bother to
lock the parent heap at all here, since we must be underneath a
BitmapHeapScan node that will be holding a suitable lock.