[#124] Create the RemoteRequest promise at construction so an unsent request can be cancelled

[vharseko]

Fixes #124

Problem

RemoteConnectionGroup.allocateRequest registers a RemoteRequest in remoteRequests before it is sent, but RemoteRequest.promise only came into existence inside the send function (getSendFunction() → apply(holder)) and was reset to null when a send failed. For the whole window between registration and a successful send getPromise() returned null, and every cancellation path dereferenced it unguarded: WebSocketConnectionGroup.shutdown(), RemoteConnectionGroup.submitRequestCancel, RemoteRequest.cancel(), plus RemoteOperationRequest.check() through getExceptionHandler().

On the client the NullPointerException escaped ClientRemoteConnectorInfoManager.doClose() after ConnectionPrincipal.close() had already flipped isRunning, so the group's delegate.close(), the private WebSocket connections, globalConnectionGroups.remove and the close listeners (ConnectionManager's registry.remove(info)) were all skipped, and a second close() was a no-op. The server side has the same loop in OpenICFWebSocketApplication.close() / OpenICFWebSocketCreator.close(), where the NPE would stop the remaining groups from shutting down.

The window became reachable from user code with #104, which moved the initial CONNECTOR_INFO request from handshake() (before connectPromise resolves) to handshakeComplete() (after it): connect().getOrThrow() now returns while that request is registered but not yet sent, and close() from the caller's thread runs concurrently with the send.

Change

RemoteRequest only:

• The promise is created in the constructor and is final; getPromise() never returns null. The completion callback (removal from remoteRequests) is registered there too - PromiseImpl fires it on cancellation as well, so a request cancelled before it was sent unregisters itself.
• The send function does not send a request whose promise is already done. A request cancelled before the send hands its caller the cancelled promise (trySubmitRequest returns the request, getOrThrow throws the cancellation exception) instead of an answer that can never arrive.
• A failed send propagates without resetting anything; the group's trySendMessage retries on its next connection with the same promise.
• tryCancel(true) sends CancelOpRequest only for a delivered message, so a request cancelled before the send puts nothing on the wire. A cancel racing a send in progress is caught by a store-then-load pair on both sides (remoteCancelRequested / requestTime, deduplicated by remoteCancelSent): the cancel message is sent after the request, once, instead of ahead of it.
• tryLock not acquired within a minute now throws IllegalStateException instead of returning an unsent promise.

Tests

RemoteRequestCancelBeforeSendTest (connector-framework-rpc), deterministic through a request that blocks in createMessageElement and a holder that blocks in sendString:

• promiseExistsWhileRequestIsRegisteredButNotSent, cancelBeforeSendCancelsPromiseWithoutSendingAnything (via submitRequestCancel), groupShutdownBeforeSendCancelsPromiseWithoutSendingAnything (the shutdown() loop) and cancelDuringSendNotifiesRemoteAfterTheRequestIsDelivered failed before the change - the first three with the exact NPE / null from the issue, the last one with the cancel message overtaking the request.
• cancelAfterSendNotifiesRemote and sendFailsOverToTheNextConnectionAndCompletesNormally pin down behaviour that had to survive the change; they pass before and after.

Local runs: connector-framework-rpc 19 tests, connector-framework-server 29, connector-server-grizzly 34, connector-server-jetty 34, all green.

Compatibility

getPromise() of a registered request is a pending promise before the send rather than null; a request cancelled before its send is returned by trySubmitRequest with a cancelled promise rather than sent. Both are visible only to code that raced the send, which previously got the NPE.

Noticed, not changed

• handshakeComplete() retries the initial CONNECTOR_INFO request on any failure, including the cancellation from shutdown(), and the retry goes out over the still-open socket before principalIsShuttingDown() removes it. Pre-existing and independent of this change: handshakeComplete() resends the initial CONNECTOR_INFO request while the group is shutting down #125.
• WebSocketConnectionGroup.principals is a plain TreeSet written from handshake threads and from principalIsShuttingDown() / close() without synchronisation.