RFR: 8223260: NamingManager should cache InitialContextFactory

[Seán Coffey]

Looks like we can improve performance in this area. I've put together a 
testcase which exercises the ServiceLoader and keeps track of whether 
we're able to cache or not.

https://bugs.openjdk.java.net/browse/JDK-8223260
http://cr.openjdk.java.net/~coffeys/webrev.8223260.v1/webrev/
regards,
Sean.