Document thread-safety requirements for server contexts (#1146)

Motivation:

The properties on the server context passed to user functions aren't
thread safe and require the user to be on the correct event loop to
handle them correctly. That should be documented!

Modifications:

- More docs
- Shim through computed properties with event-loop assertions

Result:

Better docs.

Sources/GRPC/ServerCallContexts/ServerCallContext.swift

@@ -54,17 +54,37 @@ open class ServerCallContextBase: ServerCallContext {
   /// Whether compression should be enabled for responses, defaulting to `true`. Note that for
   /// this value to take effect compression must have been enabled on the server and a compression
   /// algorithm must have been negotiated with the client.
-  public var compressionEnabled: Bool = true
+  ///
+  /// - Important: This  *must* be accessed from the context's `eventLoop` in order to ensure
+  ///   thread-safety.
+  public var compressionEnabled: Bool {
+    get {
+      self.eventLoop.assertInEventLoop()
+      return self._compressionEnabled
+    }
+    set {
+      self.eventLoop.assertInEventLoop()
+      self._compressionEnabled = newValue
+    }
+  }
+
+  private var _compressionEnabled: Bool = true
 
+  /// A `UserInfo` dictionary which is shared with the interceptor contexts for this RPC.
+  ///
   /// - Important: While `UserInfo` has value-semantics, this property retrieves from, and sets a
   ///   reference wrapped `UserInfo`. The contexts passed to interceptors provide the same
   ///   reference. As such this may be used as a mechanism to pass information between interceptors
   ///   and service providers.
+  /// - Important: This  *must* be accessed from the context's `eventLoop` in order to ensure
+  ///   thread-safety.
   public var userInfo: UserInfo {
     get {
+      self.eventLoop.assertInEventLoop()
       return self.userInfoRef.value
     }
     set {
+      self.eventLoop.assertInEventLoop()
       self.userInfoRef.value = newValue
     }
   }
@@ -75,7 +95,21 @@ open class ServerCallContextBase: ServerCallContext {
 
   /// Metadata to return at the end of the RPC. If this is required it should be updated before
   /// the `responsePromise` or `statusPromise` is fulfilled.
-  public var trailers = HPACKHeaders()
+  ///
+  /// - Important: This  *must* be accessed from the context's `eventLoop` in order to ensure
+  ///   thread-safety.
+  public var trailers: HPACKHeaders {
+    get {
+      self.eventLoop.assertInEventLoop()
+      return self._trailers
+    }
+    set {
+      self.eventLoop.assertInEventLoop()
+      self._trailers = newValue
+    }
+  }
+
+  private var _trailers: HPACKHeaders = [:]
 
   public convenience init(
     eventLoop: EventLoop,

Sources/GRPC/ServerCallContexts/UnaryResponseCallContext.swift

@@ -35,7 +35,21 @@ open class UnaryResponseCallContext<Response>: ServerCallContextBase, StatusOnly
 
   /// The status sent back to the client at the end of the RPC, providing the `responsePromise` was
   /// completed successfully.
-  public var responseStatus: GRPCStatus = .ok
+  ///
+  /// - Important: This  *must* be accessed from the context's `eventLoop` in order to ensure
+  ///   thread-safety.
+  public var responseStatus: GRPCStatus {
+    get {
+      self.eventLoop.assertInEventLoop()
+      return self._responseStatus
+    }
+    set {
+      self.eventLoop.assertInEventLoop()
+      self._responseStatus = newValue
+    }
+  }
+
+  private var _responseStatus: GRPCStatus = .ok
 
   public convenience init(
     eventLoop: EventLoop,