remove idle_timeout from user defined function API

[rdallman]

this value can be set by users per function, for a service provider or installation this makes it challenging to reason about managing machine state and can lead to capacity issues. we think it would be better to leave this is a configurable option for operators, where this policy could be enforced, but an optional policy altogether (for lower values a user might want enforced, this maybe makes more sense to forgo doing that). additionally, without this option, it may be a good idea to periodically check if a function has a 'newer' version running and eliminate all older version of that function - we need stricter versioning policy in fn in order to be able to enforce that, so ignore that.

this could simply be an env var on the agent that gets plumbed into calls if it is set, and enforced the same way as now, but removing the ability to configure it on a per function basis. that would likely satisfy the requirement here. there are other possibilities, of course...