Attempt to generalize context propagation pattern

[ashamukov]

See introduction comment in the issue: ...

Implementation details:

• common part is extracted from org.springframework.security.concurrent package to org.springframework.util.concurrent (to be easy-movable to spring-core)
• security specific part is kept in org.springframework.security.concurrent
• all public classes in org.springframework.security.concurrent are kept existing, for backward compatibility reasons (e.g. DelegatingSecurityContextRunnable is not actually needed any more, and can be removed in favor of DelegatingContextRunnable, but is kept)
• SecurityContextOps implementation of ContextOps is used to incapsulate and provide security-specific behavior
• Interfaces ContextOps and SecurityContextHolderStrategy are pretty similar, but probably should be kept both (one for backward-compatibility, another - to be moved in spring-core)

Haven't done yet:

• probably it is also worth to generalize:
  • DelegatingSecurityContextAsyncTaskExecutor
  • DelegatingSecurityContextTaskExecutor
    (right now they extend specific base classes)
• doc polishing & test fixes