When running the JWK background refresh job, a client could call the refreshKeys command at the same time.

This could cause a use after free if the refreshKeysCommand function deletes the 
_keyManager just after JWKSetRefreshJob grabs a reference to compare its keys.
 
One solution could be to instead of grabbing a reference to the keys, make a copy to avoid use after free if _keyManager gets deleted in the process.

Other solution could be to make a new shared_ptr to keep the underlying object from being destroyed.