This parameter should ease the introduction of features that may later
be removed without being considered a breaking change. For features that
get promoted to either being always present or behind the feature param,
it sends a clear signal regarding when we consider a feature ready for
production. This could have been useful for HTTP/2 support for example.

The first experimental feature is the ability to drop thread pools,
which was behind the debug param.

Having EXPERIMENTAL in the code takes a lot of real estate, but exp is
already established as short for expiry.