Improve interface for adding timestamps

Provide a simpler interface for adding <delay/> and <x/> timestamps to
stanzas.  This also makes sure that only one <delay/> tag and one <x/>
tag is added to a given stanza.
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 more efficient to add the timestamp only to
those stanzas that are actually sent to the client.