Thread hang risk

I bumped into this while running 2.19 https://github.com/Automattic/simplenote-macos/pull/1103 from Xcode.

```
Thread Performance Checker: Thread running at QOS_CLASS_USER_INTERACTIVE waiting on a lower QoS thread running at QOS_CLASS_DEFAULT. Investigate ways to avoid priority inversions
PID: 18748, TID: 7522316
Backtrace
=================================================================
3 Simperium_OSX 0x00000001037c5dd4 -[_SPRRunLoopThread runLoop] + 44
4 Simperium_OSX 0x00000001037c5b18 __45+[NSRunLoop(SPRWebSocket) SPR_networkRunLoop]_block_invoke + 124
5 libdispatch.dylib 0x00000001033d699c _dispatch_client_callout + 20
6 libdispatch.dylib 0x00000001033d88bc _dispatch_once_callout + 160
7 Simperium_OSX 0x00000001037c5a74 +[NSRunLoop(SPRWebSocket) SPR_networkRunLoop] + 88
8 Simperium_OSX 0x00000001037bf544 -[SPRWebSocket openConnection] + 76
9 Simperium_OSX 0x00000001037be250 -[SPRWebSocket open] + 240
10 Simperium_OSX 0x00000001037c9904 -[SPWebSocket open] + 76
11 Simperium_OSX 0x00000001037d3e08 -[SPWebSocketInterface openWebSocket] + 672
12 Simperium_OSX 0x00000001037d3f50 -[SPWebSocketInterface start:] + 268
13 Simperium_OSX 0x0000000103776eac -[Simperium startNetworkManagers] + 992
14 Simperium_OSX 0x000000010377b1a0 -[Simperium authenticationDidSucceedForUsername:token:] + 72
15 Simperium_OSX 0x0000000103785c28 -[SPAuthenticator authenticateIfNecessary] + 1960
16 Simperium_OSX 0x000000010377b580 -[Simperium authenticateIfNecessary] + 260
17 Simperium_OSX 0x0000000103778670 -[Simperium startWithAppID:APIKey:] + 592
18 Simperium_OSX 0x0000000103778294 -[Simperium authenticateWithAppID:APIKey:window:] + 304
19 Simplenote 0x0000000102a812fc -[SimplenoteAppDelegate applicationWillFinishLaunching:] + 364
20 CoreFoundation 0x0000000183f1b180 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ + 148
21 CoreFoundation 0x0000000183fb6eec ___CFXRegistrationPost_block_invoke + 88
22 CoreFoundation 0x0000000183fb6e34 _CFXRegistrationPost + 440
23 CoreFoundation 0x0000000183eec4cc _CFXNotificationPost + 704
24 Foundation 0x0000000184e4a8f4 -[NSNotificationCenter postNotificationName:object:userInfo:] + 88
25 AppKit 0x0000000187139584 -[NSApplication finishLaunching] + 248
26 AppKit 0x0000000187139270 -[NSApplication run] + 252
27 AppKit 0x0000000187110794 NSApplicationMain + 880
28 Simplenote 0x0000000102a81040 main + 36
29 dyld 0x0000000183aeff28 start + 2236
```

Reproduce the warning by running version 2.19 from Xcode, then trace the startup path from SimplenoteAppDelegate applicationWillFinishLaunching: through Simperium startNetworkManagers and SPWebSocketInterface start:. Inspect SPRWebSocket openConnection and the SPR_networkRunLoop creation shown in the backtrace. Done means the reported priority-inversion or thread-hang risk is resolved and the warning no longer appears during launch.

Written by the indexing model from the issue text.