QDBusMetaTypeId: don't cache the result of qMetaTypeId<>() in static ints

QDBusMetaTypeId: don't cache the result of qMetaTypeId<>() in static ints

There's not much point in caching the result of qMetaTypeId<>,
because it's already internally memoised.

In addition, the code that initialised the static int caches wasn't
protected against concurrent access under the assumption that the
operations performed were thread-safe.

That is true for most of them, but not for the stores to the static ints,
which race against each other:

   // Thread A               // Thread B
   r1 = initialized /*=false*/
                             r1 = initialized /*=false*/
   r2 = qMetaTypeId<...>();
                             r2 = qMetaTypeId<...>();
   message = r2;             message = r2; // race, ditto for all other ints

To fix, turn the ints into inline functions that just call the respective
qMetaTypeId<>() function.

Change-Id: I5aa80c624872c3867232abc26ffdcde70cd54022
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>