cqlsh: flush capture output after every command.

Patch by paul cannon, reviewed by brandonwilliams for CASSANDRA-4073

bin/cqlsh

@@ -1051,6 +1051,7 @@ class Shell(cmd.Cmd):
             self.print_count_result(self.cursor)
         elif self.cursor.description is not _VOID_DESCRIPTION:
             self.print_result(self.cursor)
+        self.flush_output()
         return True
 
     # these next two functions are not guaranteed perfect; just checks if the
@@ -2539,6 +2540,9 @@ class Shell(cmd.Cmd):
             out = self.query_out
         out.write(self.applycolor(str(text), color) + ('\n' if newline else ''))
 
+    def flush_output(self):
+        self.query_out.flush()
+
     def printerr(self, text, color=RED, newline=True, shownum=None):
         if shownum is None:
             shownum = self.show_line_nums