SHOWPLAN related changes

Reworked the fix for bug 1392522 - mxosrvr core dumped doing showplan
(with N.E. enabled), plus other related code changes.
1) The native code (also known as native expression), if generated, is
stored in the expression's constant area. SHOWPLAN will dump the native
code in the assembly format. The display can be disabled by CQD
PCODE_NE_IN_SHOWPLAN to "OFF". It is "ON" by default. This part had been
reviewed by Jim Capps and Mike Hanlon.
2) Add several SHOWPLAN statements in core/TEST019 without logging the
output. This is to ensure no core generated when getting executor
operator (TDBs) info via SHOWPLAN.
3) A temporary fix to ComTdbHbaseAccess::displayRowId(). The current way
of retrieving begin or end row IDs from the HbaseScanRows for SHOWPLAN
does not match with the way those row IDs are generated (see
HbaseAccess::genListsOfRows), causing core dumps in some cases.

Change-Id: Id33e8c854452c03e15c4bd21f668f123180d9471