python/pgq: relaxed event handling

python/pgq: relaxed event handling

.tag_done() call is no more required.  Events are by default in
'done' state.

In 2.x events were in 'retry' state by default, which was very bad
idea in retrospect.  Changing them to 'untagged' and still requiring
tag_done() does not seem too good either.  Original reasoning was to
detect and survive errors in scripts, but the result was only
confusion to everybody.

So instead of assuming that script may be buggy, now we assume
that script knows what it does.  And only by explicit action
can they be tagged as retry.