Use autoload to be threadsafe

[kirs]

The current implementation of const_missing is not thread safe and it triggers exceptions like:

NoMethodError: undefined method `implementation' for HTTP::CookieJar::AbstractStore:Class
  /app/data/bundler/ruby/2.6.0/gems/http-cookie-1.0.3/lib/http/cookie_jar.rb:38:in `get_impl'
  /app/data/bundler/ruby/2.6.0/gems/http-cookie-1.0.3/lib/http/cookie_jar.rb:74:in `initialize'
  /app/data/bundler/ruby/2.6.0/gems/rest-client-2.1.0/lib/restclient/request.rb:339:in `new'
  /app/data/bundler/ruby/2.6.0/gems/rest-client-2.1.0/lib/restclient/request.rb:339:in `process_cookie_args!'
  /app/data/bundler/ruby/2.6.0/gems/rest-client-2.1.0/lib/restclient/request.rb:86:in `initialize'
  /app/data/bundler/ruby/2.6.0/gems/rest-client-2.1.0/lib/restclient/request.rb:63:in `new'
  /app/data/bundler/ruby/2.6.0/gems/rest-client-2.1.0/lib/restclient/request.rb:63:in `execute'
  /app/data/bundler/ruby/2.6.0/gems/rest-client-2.1.0/lib/restclient/resource.rb:51:in `get'

I tried to make that const_missing thread safe by using a mutex but I don't think it's possible because of circular dependencies.

A colleague of mine suggested that I leverage autoload which is thread safe.