Core/Misc: Replace tc's optional with boost::optional. (very partial cherry-pick)

* benefits from empty optimization (objects are only constructed if needed).
* supports r-value references (move semantics) (boost >= 1.56.0).
* preparation for c++14/17's std::optional and std::none_t.
* add move constructor to CompactArray.

(cherry picked from commit 800d5d8939)

src/common/Common.h

@@ -44,6 +44,8 @@
 #include <cerrno>
 #include <csignal>
 
+#include <boost/optional.hpp>
+#include <boost/utility/in_place_factory.hpp>
 #include <boost/functional/hash.hpp>
 
 #include "Debugging/Errors.h"
@@ -152,6 +154,10 @@ typedef std::vector<std::string> StringVector;
 
 #define MAX_QUERY_LEN 32*1024
 
+//! Optional helper class to wrap optional values within.
+template <typename T>
+using Optional = boost::optional<T>;
+
 namespace Trinity
 {
     //! std::make_unique implementation (TODO: remove this once C++14 is supported)