Roster's INSTANCE map has a weak reference to XMPPConnection, which has
a field of RosterEntry(ies), and RosterEntry has a strong reference to
XMPPConnection resulting in a cycle which is preventing XMPPConnection
from getting gc'ed.
SMACK-659.
Use final where possible, add javadoc. Fix duplicate comment in
Roster.
Use createPacketCollectorAndSend(packet).nextResultOrThrow when
possible, and only set the entry name if the IQ set is successful.