420273 Close JDBC statements to prevent database resource leakages.

Make JDBCSessionManager and JDBCSessionIdManager explicitly close JDBC
statements.

When using a connection pool, not closing JDBC Statements and
PreparedStatements lead to cursor leakages in the database.

ResultSets are still not explicitly closed as they are closed automatically
when their statement is closed.

Signed-off-by: Stig Inge Lea Bjørnsen <stiginge@pvv.org>