NOT12 — Filter evaluation in pull mode

Summary: It is unclear from the spec when filters are evaluated in pull mode. For
suppliers there"s no problem as the filter is evaluated when the event
is received. On the consumer side, the filter can either be evaluated
when the event is entered into an internal queue (i.e. when the supplier
proxy "receives" the event) or when the consumer decides to pull the
event.

This is only a problem when a filter constraint contains the $curtime
reserved run-time variable. Perhaps the most intuitive for a user is
filter evalution at pull time? However, the implications can be quite
significant for sequence type consumers when invoking the
try_pull_structured_events operation. The proxy will have to match all
required events against the filters - and if there"s not enough events
all filter results must be discarded.

Added 2 sentences to 5th paragraph after constraints examples in section 2.3