Remove exception wrappers from per_operation.rb
Remove exception wrappers from per_operation.rb

I started out reverting the changes to this file back to the version
@ixti bisected as being the last good one.

I left in the *actual* code that replaced IO.select with io/wait. And
I seem to have come upon the same thing @ixti did here:

https://github.com/httprb/http/commit/9e1e100d89e365600010859ea8c87400a766c29b

I'll have to check the behavior on JRuby, and am uncertain what sequence
of events causes the rescue_readable call to be an issue, but I think this
both lets us keep io/wait and at least has the same semantics as before.

httprb/http,Connorhd/http.rb,pezra/http.rb,httprb/http.rb