Add support for SQL:2008 <result offset clause> and <fetch first clause> to limit result set cardinality

SQL 2008 has added new syntax to support a direct way to limit the
returned set of rows in a result set. This allows an application to
retrieve only some rows of an otherwise larger result set, similar to
the popular LIMIT clauses use in some databases.

Up till now, in Derby (and SQL) we have had to use the ROW_NUMBER()
function in a nested subquery to achieve the effect of the <fetch
first clause>, cf. DERBY-2998, a method which is rather more indirect
and still not efficient (DERBY-3505), and primarily intended for OLAP
functionality, perhaps.

There has been no direct way to achieve the effect of the <result
offset clause> via SQL.

Syntax (cf. SQL 2008, section 7.13):
<result offset clause> ::= OFFSET <n>

ONLY

where <n> is an integer. The two clauses syntactically follow the ORDER BY
clause in the grammar.

Note that both ORDER BY and the new clauses above are allowed also in
subqueries in the new version of the SQL standard (section 7.13). I
only propose to include this at the top level in DERBY for now. (ORDER
BY is presently also not allowed in subqueries in Derby since SQL
didn't allow for this until SQL 2008 either).