Move C++ code out of C headers

The additions into the server.h header used C++ language which caused C
programs to fail to compile. Moved the implementation of the EMAverage
class into the private Server class in the server.hh header and exposed it
via functions in the server.h header. Also temporarily moved
almost_equal_server_scores into the public server.hh as there is no
service.hh header.

include/maxscale/service.h

@@ -68,18 +68,6 @@ typedef struct server_ref_t
     bool                 active;        /**< Whether this reference is valid and in use*/
 } SERVER_REF;
 
-/** Returns true if the two server "scores" are within 1/(see code) of each other.
- *  The epsilon needs tweaking, and might even need to be in config. This
- *  function is important for some compares, where one server might be only
- *  marginally better than others, in which case historical data could determine
- *  the outcome.
- */
-inline bool almost_equal_server_scores(double lhs, double rhs)
-{
-    constexpr double div = 100;     // within 1% of each other.
-    return std::abs(lhs - rhs) < std::abs(std::max(lhs, rhs)) * (1 / div);
-}
-
 /** Macro to check whether a SERVER_REF is active */
 #define SERVER_REF_IS_ACTIVE(ref) (ref->active && server_is_active(ref->server))