MULE-7050: MuleApplicationClassLoader loadClass() method not synchronized

_Adding synchronizing FineGrainedControlClassLoader#loadClass(String,boolean)
_Removing  MuleApplicationClassLoader#loadClass(String, boolean) override
_Adding test to verify the fix
_Renaming FineGrainedControlClassLoaderTest to FineGrainedControlClassLoaderTestCase
(cherry picked from commit a2f0093)
(cherry picked from commit a72a34d)
(cherry picked from commit 8a193b2)
MULE-7087: NullSessionHandler - Empty Mule Session header
(cherry picked from commit 879eac2)
(cherry picked from commit 5d6006c)
(cherry picked from commit 05e8031)