The flag was mainly used to add a Rack::Lock middleware to
the stack, but the only scenario the lock is desired is in
development.
If you are deploying on a not-threaded server, the Rack::Lock
does not provide any benefit since you don't have concurrent
accesses. On the other hand, if you are on a threaded server,
you don't want the lock, since it defeats the purpose of using
a threaded server.
If there is someone out there, running on a thread server
and does want a lock, it can be added to your environment
as easy as: `use Rack::Lock`