Fix DBCP-369
Fix threading issue when using multiple instances of the SharedPoolDataSource concurrently.
I can't reproduce this but the issue can be seen from looking at the code.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/dbcp/branches/DBCP_1_5_x_BRANCH@1562992 13f79535-47bb-0310-9956-ffa450edef68