[Seaside] handler reuse bug? [2.3]

[Michal]

> Hm, that's scary.  I can't think of anything it would be doing to
> reuse handlers; it just creates new ones as needed.

ok, so my guess is wrong (it was partly based on the fact that you do
reuse html canvases and I was bitten by that in the past (my fault)).

> Have you looked into this any further?

Not much - except that I discovered that it was all due to one runaway
session spawning new handlers over and over again, for reasons I don't
understand. I had to get the image back into a working state fairly
quickly during a stressed moment so I just terminated that session and
that was that.

It is still unsettling that keysByHandler and handlersByKey can
mismatch in this way...