Fix race on connected

[danielrhammond]

doDisconnect(_:) gets called from a background thread as part of the operation queued in dequeueWrite(_,code:completion:) when a connection failure triggers and there's no synchronization between its setting of connected and the several places the mainthread reads its value through isConnected.

Here's an example thread sanitizer report I got by killing the server while a test app was connected.

==================
WARNING: ThreadSanitizer: data race (pid=15356)
  Write of size 1 at 0x7b5800002348 by thread T12:
  * #0 WebSocket.connected.setter WebSocket.swift (Starscream:x86_64+0xc3d5)
    #1 WebSocket.doDisconnect(_:) WebSocket.swift:930 (Starscream:x86_64+0x2f363)
    #2 closure #1 in WebSocket.dequeueWrite(_:code:writeCompletion:) WebSocket.swift:904 (Starscream:x86_64+0x2e98e)
    #3 partial apply for closure #1 in WebSocket.dequeueWrite(_:code:writeCompletion:) WebSocket.swift (Starscream:x86_64+0x3ada4)
    #4 thunk for @callee_owned () -> () WebSocket.swift (Starscream:x86_64+0x739c)
    #5 __NSBLOCKOPERATION_IS_CALLING_OUT_TO_A_BLOCK__ <null>:6963056 (Foundation:x86_64+0x441a8)
    #6 _dispatch_client_callout <null>:6963056 (libdispatch.dylib:x86_64+0x178b)

  Previous read of size 1 at 0x7b5800002348 by thread T2:
  * #0 WebSocket.connected.getter WebSocket.swift (Starscream:x86_64+0xc354)
    #1 WebSocket.isConnected.getter WebSocket.swift:136 (Starscream:x86_64+0xbd46)
    #2 WebSocket.write(string:completion:) WebSocket.swift:231 (Starscream:x86_64+0x1123b)
    #3 Controller.sendTimestamp() ViewController.swift:19 (Starscream Crash Test:x86_64+0x100002cec)
    #4 closure #1 in Controller.websocketDidConnect(socket:) ViewController.swift:28 (Starscream Crash Test:x86_64+0x1000032b3)
    #5 partial apply for closure #1 in Controller.websocketDidConnect(socket:) ViewController.swift (Starscream Crash Test:x86_64+0x10000331d)
    #6 thunk for @callee_owned () -> () ViewController.swift (Starscream Crash Test:x86_64+0x1000033ac)
    #7 __tsan::invoke_and_release_block(void*) <null>:6963056 (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x6565b)
    #8 _dispatch_client_callout <null>:6963056 (libdispatch.dylib:x86_64+0x178b)

  Issue is caused by frames marked with "*".

  Location is heap block of size 664 at 0x7b5800002100 allocated by main thread:
    #0 calloc <null>:6963088 (libclang_rt.tsan_osx_dynamic.dylib:x86_64h+0x481b2)
    #1 class_createInstance <null>:6963088 (libobjc.A.dylib:x86_64h+0x6e9c)
    #2 Controller.init() ViewController.swift:8 (Starscream Crash Test:x86_64+0x100002459)
    #3 Controller.__allocating_init() ViewController.swift (Starscream Crash Test:x86_64+0x100001fb5)
    #4 ViewController.viewDidLoad() ViewController.swift:55 (Starscream Crash Test:x86_64+0x10000471f)
    #5 @objc ViewController.viewDidLoad() ViewController.swift (Starscream Crash Test:x86_64+0x100004a30)
    #6 -[NSViewController _sendViewDidLoad] <null>:6963088 (AppKit:x86_64+0x961ca)
    #7 start <null>:6963088 (libdyld.dylib:x86_64+0x5234)

  Thread T12 (tid=344393, running) is a GCD worker thread

  Thread T2 (tid=344370, running) is a GCD worker thread

SUMMARY: ThreadSanitizer: data race WebSocket.swift in WebSocket.connected.setter
==================

[daltoniam]

Thanks! This has been a problem for a some time, hadn't gotten around to fixing it.