Merge commit '9032c6cbe74ccf7e8963755501e7e6cc473ae471' into cluster

This is a rather involved merge, since the cluster changes interact
fairly badly with the refactoring we're merging:

The new code wants the search to know nothing at all about the UCI protocol,
it just calls a callback with some worker information (including a Position),
which then transforms that information into a InfoFull which is then sent
through another callback, upon which UCIEngine converts it to UCI and writes
it to stdout.

On the other hand, the MPI information wants to exchange PVs between workers
at this point, and it wants to use the UCI string as the primary medium of
exchange. Position is a lot of work to serialize, so we choose a middle road;
we capture the InfoFull by switching out the callback, serializes it, does the
MPI exchange on that, unserializes it and then continues with the print callback
(if there is new information).

src/thread.h

@@ -83,7 +83,7 @@ class ThreadPool {
 
     void start_thinking(const OptionsMap&, Position&, StateListPtr&, Search::LimitsType);
     void clear();
-    void set(Search::SharedState);
+    void set(Search::SharedState, Search::SearchManager::UpdateContext&);
 
     Search::SearchManager* main_manager();
     Thread*                main_thread() const { return threads.front(); }