sync service: add support for APNS

[tburgin]

This CL lays the groundwork for using APNS with a sync server. Baring hacky workarounds, APNS must be registered by a process running in the GUI domain. The sync service runs in the global domain and cannot register for APNS itself. Most of the complexity of this CL is to handle APNS registration from the GUI process, while continuing to use the sync service for communications with the sync server.

sequenceDiagram
    box Mac
    participant GUI 2
    participant GUI
    participant Daemon
    participant Sync Service
    end
    box Network
    participant Sync Server
    participant Apple APNs
    end

    GUI->>Apple APNs: Register for APNS
    Sync Service->>Daemon: Get APNS Token
    Daemon->>GUI: Get APNS Token
    GUI->>GUI: Wait for APNS Registration
    Apple APNs-->>GUI: APNS Token
    GUI-->>Daemon: Token in reply block
    Daemon-->>Sync Service:  Token in reply block
    Sync Service->>Sync Server: Token 1
    Sync Server->>Apple APNs: Message Targeting Token 1
    Apple APNs->>GUI: Message
    GUI->>Sync Service: Forward Message

    GUI 2->GUI 2: Fast User Switch & APNS Register
    GUI 2->>Sync Service: Token Changed
    Sync Service->>Daemon: Get APNS Token
    Daemon->>GUI 2: Get APNS Token
    GUI 2-->>Daemon: Token in reply block
    Daemon-->>Sync Service:  Token in reply block
    Sync Service->>Sync Server: Token 2
    Sync Server->>Apple APNs: Message Targeting Token 2
    Apple APNs->>GUI 2: Message
    GUI 2->>Sync Service: Forward Message

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