Accumulated SRT fixes

Improvements to the SRT plugin extracted from LTN's patchset:

• srt: Replace stats accumulation with naive byte counting
  srt_bstats cannot be used to get the stats of closed connections, so the best we can do is keep the running count ourselves.
• srt: Simplify socket stats
  Don't hide stats depending on whether we're a sending or receiving socket. While we're here, add some more debug logs.
• srt: Clean up error handling

  • Make the srt_epoll_wait loops more uniform.

  • Error only via GError when possible; let the element send the error message. Avoids a second error message.

  • Return 0 when cancelled. Avoids an error message from the element.

  • Don't send an error message from send_headers when we're a server sink.

• srt: Clean up poll/sock lifecycle
  Make sure srtobject->poll_id is never invalid as long as srtobject exists. Only remove our caller socket from it when the socket becomes invalid.
• srt: Use simpler list operations for callers
  Avoid g_list_append and g_list_remove (which have to scan the list) and replace them with g_list_prepend and g_list_delete_link.
• srt: Remove callers for which srt_bstats fails
  This keeps them from accumulating in the element and in the stats while the sink is not being fed, as long as we at least periodically grab stats.