Random: Replaced drand48 with an in-class implementation

This is faster than the library functionality that it replaces, as it
allows the compiler to do inlining. It also does not utilise any static
state so generators do not interfere with each other. It is also faster
than the the array lookup in cachedRandom. The cachedRandom class
therefore offers no advantage over Random and has been removed.

applications/test/fieldMapping/Test-fieldMapping.C

@@ -163,7 +163,7 @@ int main(int argc, char *argv[])
         }
 
         // Remove face
-        label candidateFacei = rndGen.integer(0, mesh.nInternalFaces()-1);
+        label candidateFacei = rndGen.sampleAB<label>(0, mesh.nInternalFaces());
         Info<< "Wanting to delete face " << mesh.faceCentres()[candidateFacei]
             << nl << endl;