Delay in connecting when first of multiple providers is down can cause performance issue

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.