rack.multithread is only true for Thread* models

rack.multithread is only true for Thread* models

Enabling thread-safe or thread-aware code paths in applications
may even be dangerous in some cases and cause deadlocks in code
that otherwise does not expect threads.  This is especially true
of the Revactor case where being a "drop-in" replacement for IO
routines is dangerous if a mutex is held while an Actor performs
a "blocking" I/O operation.

Basically start to assume that anybody writing an app using
Rev or Revactor already takes Rev/Revactor concurrency into
account and won't need the rack.multithread flag set to do
special things.