Process certificate signing request in the right event loop

Summary:
To ensure that no deadlocks take place, it is important that there are no re-entrant calls from within the callbacks or event handlers.

For the most part, this was already the case. Event and message handlers run critical sections into a Folly event scheduler.

The only exception was the sendExpectResponse used during the certificate exchange. Once the response was received, the non-secure socket was disconnected.
The solution was to put that operation in the Folly event scheduler as it should've been from the beginning.

changelog: Certificate signing request response to be processed on the right event loop.

Reviewed By: fabiomassimo

Differential Revision: D35548148

fbshipit-source-id: cea2476ad66137f376acda66cdbc27801c0c47e1

xplat/Flipper/FlipperConnectionManagerImpl.h

@@ -82,7 +82,11 @@ class FlipperConnectionManagerImpl : public FlipperConnectionManager {
   bool connectAndExchangeCertificate();
   bool connectSecurely();
   bool isCertificateExchangeNeeded();
-  void requestSignedCertFromFlipper();
+  void requestSignedCertificate();
+  void processSignedCertificateResponse(
+      std::shared_ptr<FlipperStep> gettingCertificateStep,
+      std::string response,
+      bool isError);
   bool isRunningInOwnThread();
   void reevaluateSocketProvider();
   std::string getDeviceId();