Tweak a few things about the threadsafe let solution from #1858.

Created by: myronmarston

1). Remove --threadsafe CLI option.

In general, we don't expose every config option via the CLI. We want to make it easy for users to run rspec --help and find the options that they are commonly going to want to customize for a particular CLI run. I don't think --threadsafe is one of those -- it's more something that'll be turned off globally for the project or not touched at all, and as such, it adds noise to the --help output to include it.

2). Extract Mutex class into its own file.

• Remove need for use of MUTEX over Mutex.
• No reason to force Ruby to parse the code for 1.8.7 Mutex implementation on other Rubies.

3). Remove threadsafe scenario.

A note about threadsafey is sufficient for the docs. Having a full threadsafety example spec in the scenario is more detail than users are likely to want to read.

/cc @JoshCheek