org.jnp.interfaces.NamingContext.checkRef always loops through the provider list in order,
trying to connect to the first before checking for a cached connection to a later provider.

This affects performance because it can cause a delay on every context created, waiting for a timeout on a node that is down.

For instance, the following takes a very long time when node1 is down.

...
env.put ( Context.PROVIDER_URL, "node1:1100,node2:1100" );
for ( int i = 0; i < 1000; ++ i )

If checkRef looked for a cached connection to any of the listed providers before trying to connect to an uncached one,
there would only be a timeout delay on the very first connection attempt.