MULE-7080: Race condition checking file attributes on SFTP transport

_ Moving initialization of static fields right before the SFTP server start
(cherry picked from commit 4a3d127)
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)
MULE-7087: NullSessionHandler - Empty Mule Session header
(cherry picked from commit 879eac2)