BUG: DTLS deferred/observe responses sent as plaintext

[cvik]

Severity: HIGH (from external code review)

drainDeferred() and drainNotifications() use send_data() which queues raw UDP sends. For requests that arrived over DTLS, follow-up traffic (deferred responses, observe notifications) must be encrypted.

Currently, secure requests can produce plaintext follow-up traffic — a correctness and security bug.

Fix: Track whether the original request was DTLS. Use send_dtls_packet() for secure sessions.

[cvik]

Fixed in PR #89 — sendMaybeEncrypted checks for DTLS session in drain paths.