Rename SharedMutex and introduce Mutex

[jac3km4]

I was looking into the RTTI and there appear to be 4 types of locks:

• spin lock (currently named SpinLock in the SDK)
• shared spin lock (currently named SharedMutex in the SDK)
• mutex (uses critical section, does not exist in the SDK)
• shared mutex (uses srwlock, does not exist in the SDK)

In this PR I do a little bit of cleanup:

• rename SharedMutex to SharedSpinLock, because that's the more appopriate name, in c++ a shared mutex corresponds to a srwlock on Windows, not a spin lock
• introduce Mutex
• update the RTTI system to make use of Mutex and SharedSpinLock

I didn't add the rwlock because I don't think it's needed anywhere in the SDK right now.

[wopss]

Thanks! I added my comments.