Enable add/remove of multiple ConnectionListeners per Connection

[davidmcote]

This patch addresses #860.

Late and multiple bindings of ConnectionListeners to Connections enable business logic to listen for events without needing to construct their own Connection object.

This benefits applications which separate their NATS Connection factory from their business logic, but whose business logic's resource's lifecycle are coupled to the state of NATS Connection's. Eg: They wish to clean up on connection CLOSED events without polling or passing around a separate observable mechanism.

[davidmcote]

FYI: I couldn't seem to get a clean test run on my system before setting out to develop this patch.

My testing here focused on ensuring ConnectionListenerTest remained 100% passing.

Is it typical to see a subset of the unit test suite failing intermittently with issues launching nats-server processes?

java.lang.IllegalStateException: Failed to run [nats-server --config /var/folders/37/vq3x8fg51gvfmm3y8mn92tjc0000gn/T/nats_java_test5680736364639790246.conf -js]
	at nats.io.NatsServerRunner.<init>(NatsServerRunner.java:411)
	at nats.io.NatsServerRunner.<init>(NatsServerRunner.java:106)
	at io.nats.client.NatsTestServer.<init>(NatsTestServer.java:46)
	at io.nats.client.utils.TestBase.runInServer(TestBase.java:100)
	at io.nats.client.utils.TestBase.runInJsServer(TestBase.java:88)
	at io.nats.client.impl.JetStreamGeneralTests.testMoreCreateSubscriptionErrors(JetStreamGeneralTests.java:903)
. . .

[scottf]

Is it typical to see a subset of the unit test suite failing intermittently with issues launching nats-server processes?

Yes. We could verify the server is ready I suppose...

[scottf]

LGTM

[davidmcote]

Thanks for the review Scott :)