Refactor network layer to use streams

Summary:
The network layer now has a `Session` that handles all things that should be
done before the `Execute` method is called on sessions. Also, all sessions
now communicate using streams instead of holding the input buffer and writing
to the `Socket`. This design will allow implementation of a SSL middleware.

Reviewers: buda, dgleich

Reviewed By: buda

Subscribers: pullbot

Differential Revision: https://phabricator.memgraph.io/D1314

tests/concurrent/network_session_leak.cpp

@@ -22,7 +22,7 @@ TEST(Network, SessionLeak) {
 
   // initialize server
   TestData session_data;
-  ServerT server(endpoint, session_data, false, "Test", 2);
+  ServerT server(endpoint, session_data, -1, "Test", 2);
 
   // start clients
   int N = 50;