LDA: Turn off optimizer only when GPDB is 4.3

Pivotal Tracker: 58839874

Commit 14e2d74 added a fix for LDA by turning-off ORCA. The commit, however,
didn't add a check for GPDB 4.3 and hence led to issues with postgres and GPDB
4.1 and 4.2. This commit fixes that problem by checking if the GPDB version
is specifically 4.3 before turning the optimizer off.