Throw exceptions on RPC failure and Distributed error handling

Summary:
This diff changes the RPC layer to directly return `TResponse` to the user when
issuing a `Call<...>` RPC call. The call throws an exception on failure
(instead of the previous return `nullopt`).

All servers (network, RPC and distributed) are set to have explicit `Shutdown`
methods so that a controlled shutdown can always be performed. The object
destructors now have `CHECK`s to enforce that the `AwaitShutdown` methods were
called.

The distributed memgraph is changed that none of the binaries (master/workers)
crash when there is a communication failure. Instead, the whole cluster starts
a graceful shutdown when a persistent communication error is detected.
Transient errors are allowed during execution. The transaction that errored out
will be aborted on the whole cluster. The cluster state is managed using a new
Heartbeat RPC call.

Reviewers: buda, teon.banek, msantl

Reviewed By: teon.banek

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D1604

tests/concurrent/network_server.cpp

@@ -33,6 +33,10 @@ TEST(Network, Server) {
 
   // cleanup clients
   for (int i = 0; i < N; ++i) clients[i].join();
+
+  // shutdown server
+  server.Shutdown();
+  server.AwaitShutdown();
 }
 
 int main(int argc, char **argv) {