Set up sensible limits for PVF execution and preparation timeouts

[s0me0ne-unkn0wn]

In #1774, we introduced executor parameters coherence checks. For most parameters, it includes checking if the parameter's value is within some sensible range. However, we failed to establish such ranges for preparation timeouts (for pre-checking and actual preparation, separately) and execution timeouts (for backing and approvals, separately). What is checked right now is just that the approval timeout is longer than the backing one, and the preparation timeout is longer than the pre-checking one.

It would be good to restrict their values further to some range that makes sense. E.g., zero preparation timeout does not make sense, as well as the backing timeout longer than the block time. It should be a simple change; we just need to figure out what sensible values are.

CC @eskimor @burdges @eagr