Allow for clarification of Thread Safety.

[GoogleCodeExporter]

For thread safe components, I like to document the type of thread safety. 
This information could be checked staticly.
e.g.
- The caller is responsible for thread safety. This could check the caller
actually does this is some way. e.g. has a ThreadSafety annotation.
- Locking is global by a static lock.
- Locking allows one thread at a time per instance
- Locking allows concurrent read (one thread writing)
- Locking allows concurrent reading and writing.
- State is thread local.
- The class/object is stateless.
- The thread safety cannot be determined staticly.

Currently, there is only two annotations ThreadSafe and NotThreadSafe with
no distiction, or checking of the type of thread safety.

Original issue reported on code.google.com by [email protected] on 27 Jan 2009 at 5:01