Purpose or Intent

Puma handles each request in a new thread, leading to possible thread issues
with the database connection pool, ActiveSupport::Dependencies interlock and our
locks, or thread unsafe code in our application.

Thin, while it may be run in threads via --threaded, does not use threads by default.
See here

This patch allows for thin 1.7.0+ and gives us an option to use either puma or
thin. Note, earlier versions are not compatible with rack 2.

We still default to puma.
Steps for Testing/QA

Note: Testers/users, the proctitle for UI/Webservice and web socket workers will
look different in ps, top, etc. if you use thin instead of puma. Puma configures
it's own proctitle and we configure the parts that we can. Thin does not, so it
will look like all the other workers.

For example:

thin:
43177 ttys002 0:08.20 MIQ: MiqUiWorker id: 158, uri: http://0.0.0.0:3000

puma:
43871 ttys004 0:00.68 puma 3.3.0 (tcp://0.0.0.0:3000) [MIQ: Web Server Worker]