Allow Monotonic to use custom types with to_string defined

ThrowExceptionPolicy tries to log the current and new values when the
Monotonic Invariant fails. Rather than specifying

 std::to_string(value)

use

 using std::to_string
 ... to_string(value) ...

to allow the use of custom defined `to_string` (e.g., through ADL)

Change-Id: Ibef1671f72adcec92fec2dcf28978583f2cadc14
Reviewed-on: http://review.couchbase.org/111811
Reviewed-by: Dave Rigby <[email protected]>
Tested-by: Build Bot <[email protected]>

Author: jameseh96

engines/ep/src/monotonic.h

@@ -38,11 +38,12 @@ struct ThrowExceptionPolicy {
     void nonMonotonic(const std::string& label,
                       const T& curValue,
                       const T& newValue) {
+        using std::to_string;
         throw std::logic_error(
                 std::string("Monotonic<") + typeid(T).name() + "> (" + label +
-                ") invariant failed: new value (" + std::to_string(newValue) +
+                ") invariant failed: new value (" + to_string(newValue) +
                 ") breaks invariant on current value (" +
-                std::to_string(curValue) + ")");
+                to_string(curValue) + ")");
     }
 };