enable detach & attach EventBase ops on folly::coro::Transport

Summary:
The high-level goal here is to enable sychronously detaching current evb and attaching a new evb during an in-progress ReadCallback.

This diff adds a few things:
* Adds the necessary virtual functions for a supporting implementation to override
* Changes the visibility of member variables, namely EventBase and AsyncTransport, from private to protected. The overriding implementation will require access to these members to facilitate evb migration

Reviewed By: afrind

Differential Revision: D61883812

fbshipit-source-id: 42309f6f482094703df72e252ff5ab241a030c42

folly/io/coro/Transport.h

@@ -69,6 +69,10 @@ class TransportIf {
   virtual void closeWithReset() = 0;
   virtual folly::AsyncTransport* getTransport() const = 0;
   virtual const AsyncTransportCertificate* getPeerCertificate() const = 0;
+  virtual void detachEventBase() { LOG(FATAL) << "not implemented"; }
+  virtual void attachEventBase(folly::EventBase*) {
+    LOG(FATAL) << "not implemented";
+  }
 };
 
 class Transport : public TransportIf {
@@ -146,13 +150,15 @@ class Transport : public TransportIf {
     return transport_->getPeerCertificate();
   }
 
+ protected:
+  EventBase* eventBase_;
+  AsyncTransport::UniquePtr transport_;
+
  private:
   // non-copyable
   Transport(const Transport&) = delete;
   Transport& operator=(const Transport&) = delete;
 
-  EventBase* eventBase_;
-  AsyncTransport::UniquePtr transport_;
   bool deferredReadEOF_{false};
 };