Add persistent events

[mat-hek]

Let's consider a case with DTLS handshake and SRTP encryption. We have SRTP encryptor, DTLS sink and some elements between them. Now, we send SRTP key via an upstream event from DTLS to SRTP. SRTP can start encrypting packets and everything works. But, if we'd like to connect more SRTP encryptors to the same DTLS sink, for example via a funnel, the newly added encryptors won't receive the SRTP key, as the event is already gone.

A solution to that may be to introduce persistent events, that will be stored in the context, and elements like funnel will have to send them to any newly added pad, like the caps.

[mat-hek]

Moved from #286, see the full discussion there