Allow Oracle CLOB/NCLOB/BLOB in returning

Allow Oracle CLOB/NCLOB/BLOB in returning

Fixed bug in Oracle dialect where retriving a CLOB/BLOB column via
:meth:`_dml.Insert.returning` would fail as the LOB value would need to be
read when returned; additionally, repaired support for retrieval of Unicode
values via RETURNING under Python 2.

As of yet, we still don't know how to reproduce the
ORA-24813 error indicated in the issue.

Also backporting the statement cache clear added to master
in f1e96cb087 , as we are testing in CI against two oracle
versions now there are sporadic failures that appear to be
memory related.

Fixes: #5812
Change-Id: I666f893e762dfa4d34dd2e324480565b226fb3a4
(cherry picked from commit 03179a96bfb9dd7ce17274fed44908c25229dedf)