Repair _execute_scalar for WITH_UNICODE mode

Repair _execute_scalar for WITH_UNICODE mode

cx_Oracle 5.3 seems to code this flag ON now, so
remove the warning and ensure WITH_UNICODE handling works.

Additionally, the XE setup on jenkins is having more
problems here, in particular low-connections mode is
causing cx_Oracle to fail more frequently now.  Turning
off low-connections fixes those but then we get the
TNS errors, so adding an emergency "retry" flag that
is not yet a feature available to users.  Real world
applications are not dropping/creating thousands of
tables the way our test suite is.

Change-Id: Ie95b0e697276c404d3264c2e624e870463d966d6
Fixes: #3937