Do not store SDP offer/answer for calls forever

Currently call SDP is stored in the message parameters. Once the call has ended or expired (missing call), SDP remains there. It should be removed eventually if it is not needed because it normally contains IP addresses of the users.

It is currently difficult to find unneeded SDPs because you cannot easily query parameters. One way to fix this is to create a new `calls` table for SDPs and msg_id references and store SDPs there, then cleanup it in housekeeping.