Zipkin Reporter 2.12

Zipkin Reporter 2.12 improves stability of the AsyncReporter during shutdown and with certain sender implementations.

ClosedSenderException was added to signal when a sender is used after having been closed. Previously, sender implementations used an implicit convention of throwing new IllegalStateException("closed"); to signal this, but this makes it more difficult for logic to differentiate against other IllegalStateException that may be thrown by a sender. All sender implementations going forward should throw ClosedSenderException instead.