Auto merge of #2555 - jasondavies:fix-2554, r=str4d

Fix various thread assertion errors caused during shutdown.

Cherry-picked from the following upstream PRs:

- bitcoin/bitcoin#6719
- bitcoin/bitcoin#6990
- bitcoin/bitcoin#8421
  - Second commit only in this PR
- bitcoin/bitcoin#11006

I've cherry-picked the relevant commits, along with a note in each commit referring to the original Bitcoin commit ID (and the Zcash issue numbers where applicable).  I've tested each issue with/without these patches applied.

Closes #2214, #2334, and #2554.

src/rpcserver.cpp

@@ -246,7 +246,8 @@ UniValue stop(const UniValue& params, bool fHelp)
         throw runtime_error(
             "stop\n"
             "\nStop Zcash server.");
-    // Shutdown will take long enough that the response should get back
+    // Event loop will exit after current HTTP requests have been handled, so
+    // this reply will get back to the client.
     StartShutdown();
     return "Zcash server stopping";
 }