Fix crash in CBLRemoteSession when

* The issue hapens due to the race between setting the _session to nil in the -URLSession:didBecomeInvalidWithError: delegate method and calling NSURLSession’s finishTasksAndInvalidate when closing the CBLRemoteSession. The issue is a regression of fixing #1429 (race of resetting _allRequests).

* Solution: created a strong session that will exists through the block execution.

* Note: Calling NSURLSessions’s -finishTasksAndInvalidate after the sesion is invalidated is no-ops operation (from manual testing).

#1923

Source/CBLRemoteSession.m

@@ -96,11 +96,14 @@ - (void)dealloc {
 
 
 - (void) close {
-    [_session.delegateQueue addOperationWithBlock:^{
+    // Create a strong session in block to prevent race when _session
+    // is set to nil in the -URLSession:didBecomeInvalidWithError: method.
+    NSURLSession* session = _session;
+    [_session.delegateQueue addOperationWithBlock: ^{
         // Do this on the queue so that it's properly ordered with the tasks being started in
         // the -startRequest method.
         LogTo(RemoteRequest, @"CBLRemoteSession closing");
-        [_session finishTasksAndInvalidate];
+        [session finishTasksAndInvalidate];
         _requestDelegate = nil;
     }];
 }