DERBY-6107: Connection to classpath database fails when login timeout is set

Don't cache threads used for connecting to the database when a login
timeout is set, to prevent that the connection attempt uses the wrong
context class loader.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1583304 13f79535-47bb-0310-9956-ffa450edef68