Add new configuration option for JDBC4 connection test timeout.

Some databases (e.g. Oracle) can conceal multiple server nodes behind a single connection URL. So if we timeout while trying to connect to one node, we can still hope to connect to one of the other nodes. However, this assumes that the connection timeout is smaller than BTM's connection acquisition timeout.

The JDBC4 isValid() connection test has its own timeout parameter; allow it to be configured independently.