Use native IBM DB2 11 syntax for Top-N queries

[morozov]

Q	A
Type	improvement
BC Break	no

IBM DB2 supports the MySQL syntax without additional configuration as of release 11.1 (source):

0x4000 (obsolete) This bit formerly activated LIMIT and OFFSET support, but that feature is now always active and this bit is now ignored.

UPD: it doesn't seem to support OFFSET without a LIMIT in the MySQL syntax but it supports the Oracle's one.
UPD: I also checked that IBM DB2 10.5 (implicitly supported in DBAL 3.x) also supports the FETCH n ROWS ONLY clause but doesn't support the OFFSET n ROWS clause, so this patch can be only accepted for DBAL 4.0.