!hco #15635 reorganize registration of predefined, custom HTTP primitives + other small improvements

Fixes #15635.

@richdougherty can you check if that would work for you?
Right, we need to document this and add a length validation.
Also, if you know of any case where a custom method was longer it's not a problem to increase this limit.
However, I'd like to keep it somewhat aggressive since, in many cases, this is the trigger that aborts the parse attempt on non-HTTP data coming in across the wire.