[Bug] Add the missing RTE_GRAPH_TABLE case to transformLockingClause()

Hi Hackers,

Add the missing RTE_GRAPH_TABLE case to transformLockingClause(). Without
this four
row-locking strengths applied to a GRAPH_TABLE alias triggers not give a
user friendly
error message.

Repro:

CREATE TABLE v(id int PRIMARY KEY, vname text);
CREATE PROPERTY GRAPH g VERTEX TABLES (v);
SELECT * FROM GRAPH_TABLE(g MATCH (a) COLUMNS (a.vname)) gt
FOR UPDATE OF gt;
-- ERROR: unrecognized RTE type: 8

Attached a patch that returns ERRCODE_FEATURE_NOT_SUPPORTED "FOR ... cannot
be
applied to a GRAPH_TABLE" with a position pointer, matching the convention
used by
the function/tablefunc etc. Patch includes tests for all four locking
strengths.
Since the code path looks simple we can just keep one of them as well and
trim other
tests. Thoughts?

Thanks,
Satya