Fixing shift/reduce conflicts introduced by the new system versioning
syntax.

Additionally, fixing old shift/reduce conflicts:

In PREVIOUS/NEXT being identifiers or sequence operations:

  SELECT PREVIOUS FROM t1;
  SELECT PREVIOUS VALUE FOR t1;

In TIME/DATE/TIMESTAMP being literals, functions or identifiers:

  SELECT TIMESTAMP'2001-01-01 10:20:30' FROM t1;
  SELECT TIMESTAMP('2001-01-01 10:20:30') FROM t1;
  SELECT TIMESTAMP FROM t1;