backends/oracle: support working in multithreaded programs

OCI_THREADED ensures that shared OCI data is properly protected.

OCI_ENV_NO_MUTEX avoids using mutexes when there are no concurrent
accesses to a session, which is the multithreading model that SOCI
employs.

More details:
http://docs.oracle.com/cd/B10501_01/appdev.920/a96584/oci15re4.htm
Merge pull request #31 from avg-I/oracle/multithreading

backends/oracle: support working in multithreaded programs