Nil out the `onStart` callback after using it (#1570)

Motivation:

The `ClientTransport` has an `onStart` callback which is called once the
stream has been established. For async calls this is used to notify the
writability manager that it may now start writing. The callback
references the call which holds the transport which holds the callback
forming a strong retain cycle and a slow memory leak. The transport
should break this cycle.

Modifications:

- nil out the `onStart` callback after it has been called.

Result:

Fixes a leak

Sources/GRPC/Interceptor/ClientTransport.swift

@@ -94,7 +94,7 @@ internal final class ClientTransport<Request, Response> {
   private var channel: Channel?
 
   /// A callback which is invoked once when the stream channel becomes active.
-  private let onStart: () -> Void
+  private var onStart: (() -> Void)?
 
   /// Our current state as logging metadata.
   private var stateForLogging: Logger.MetadataValue {
@@ -945,7 +945,8 @@ extension ClientTransport {
       // Messages are buffered by this class and in the async writer for async calls. Initially the
       // async writer is not allowed to emit messages; the call to 'onStart()' signals that messages
       // may be emitted. We call it here to avoid races between writing headers and messages.
-      self.onStart()
+      self.onStart?()
+      self.onStart = nil
 
     case let .message(request, metadata):
       do {