check if remote contact support esession or session. If it supports esession, it supports session but may not announce it. see #4429

For performance reasons, get_contact_iters() cannot find a contact if get_shown_groups() returns something different to what has been applied to roster model. The contact was therefore duplicated as it was believed not to be there...

Correctly remove Observer and NotInRoster contact in one special case. Does not affect pending bug. See #4524.