Introduce `Runtime` class to identify runtime proc

This will serve as the new single access point into
identifying which runtime is active.

Add Process.max_threads method

This will return the maximum concurrency for the given
runtime environment.

Revert to including `defined?` checks for Process

This is based on a reference impl by new relic which they
use to detect the current dispatcher.

Add `name` method, throw if ambiguous

This can be called from an initializer for instance.

Log the current runtime in an initializer

Add `multi_threaded?` and `app_server?` helpers

To allow easier grouping of configuration

Rename `Process` to `Runtime`

And move it into its own file.

Replace all remaining runtime checks with new API

Including a commit body because the danger bot politely asked me
to. There really is nothing else to say.

Prefer `class << self` over `instance_eval`

It seems to be the more widely adopted style.

Simplify `has_instances?` helper method

Fix rubocop offense

Remove max_threads method

This wasn't currently used anywhere and we should define this elsewhere.

Remove references to NR library

This caused some legal questions. We weren't using the instance lookup
before, so it should be OK.