Unbalanced synchronization of ResponseContext.REGISTERED_KEYS

[leventov]

To make it really thread-safe:

1. keyOf() and getAllRegisteredKeys() should be made synchronized;
2. getAllRegisteredKeys() should return a defensive copy: new TreeMap<>(REGISTERED_KEYS).values()

Relevant concurrency checklist items: https://github.com/code-review-checklists/java-concurrency#unsafe-concurrent-point-read, https://github.com/code-review-checklists/java-concurrency#unsafe-concurrent-iteration

[esevastyanov]

Agreed. There is no guarantee that registerKey would be called only from class static blocks as originally designed.