[crossrepo] Add 'timestampMillis' to RPC Event messages
[crossrepo] Add 'timestampMillis' to RPC Event messages

Java applications have 'System.currentTimeMillis()' which is currently
used in OpenVidu to timestamp all events. It's desirable to have at
least the same precision from KMS events, so the timestamps can be
matched.

Cross-repo commit:
- https://github.com/Kurento/kms-core
- https://github.com/Kurento/kurento-module-creator
- https://github.com/Kurento/doc-kurento

Kurento/kms-core,Kurento/kms-core,Kurento/kms-core,Kurento/kms-core