Make oci_execute() calls inside num_rows() non-committing. Fixes #696.

Fixes bug in Oracle driver. Calls to oci_execute() inside num_rows()
of the Oracle driver can be made non-committing to fix bug with transactions.
Since calls to oci_execute are only there to reset which row is next in line
for oci_fetch() calls, it's fine to not commit.
Merge pull request #2761 from a-krebs/2.1-stable

Make oci_execute() calls inside num_rows() non-committing. Fixes #696.