Improve interface for adding timestamps

Provide a simpler interface for adding <delay/> and <x/> timestamps to
stanzas.  This makes sure that only one <delay/> tag and one <x/> tag is
added to a given stanza.  The obsolete <x/> tag could now easily be
omitted by editing only the new jlib:add_delay_info/4 function.
XEP-0198: Add timestamp to resent stanzas

When an unacknowledged stanza is resent from the Stream Management
queue, add a timestamp so that the receiving client can display the time
at which the stanza was originally sent.
Let CSI code add timestamp at later point in time

Don't tag presence stanzas with a timestamp *before* putting them into
the CSI queue.  As all but the latest presence stanzas end up being
dropped from the queue, it's slightly more efficient to add the
timestamp only to those stanzas that are actually sent to the client.
Remove XEP-0091 (Legacy Delayed Delivery) support

XEP-0091 has been deprecated in favor of XEP-0203 in 2007, and the
status has been changed to "Obsolete" in 2009.