cql3: Add TRUNCATE TABLE alias for TRUNCATE

CQL 3.2.1 introduces a "TRUNCATE TABLE X" alias for "TRUNCATE X":

  4e3555c1d9

Fix our CQL grammar to also support that.

Please note that we don't bump up advertised CQL version yet because our
cqlsh clients won't be able to connect by default until we upgrade them
to C* 2.1.10 or later.

Fixes #576

Signed-off-by: Pekka Enberg <penberg@scylladb.com>

cql3/Cql.g

@@ -856,7 +856,7 @@ dropIndexStatement returns [DropIndexStatement expr]
   * TRUNCATE <CF>;
   */
 truncateStatement returns [::shared_ptr<truncate_statement> stmt]
-    : K_TRUNCATE cf=columnFamilyName { $stmt = ::make_shared<truncate_statement>(cf); }
+    : K_TRUNCATE (K_COLUMNFAMILY)? cf=columnFamilyName { $stmt = ::make_shared<truncate_statement>(cf); }
     ;
 
 #if 0