rtpjitterbuffer: fix stalling when resetting timers

When calling gst_rtp_jitter_buffer_reset you pass in a seqnum.

This is considered the starting-point for a new stream.

However, the old behavior would unref this buffer, basically lying to the thread that is pushing out buffers saying that it can expect this buffer, when it would never arrive. The resulting effect being no more buffer pushed out of the jitterbuffer, and it would buffer incoming data indefinitely.

By instead inserting the buffer in the gap_packets queue, the _reset() function will take responsibility for using that as the first buffer of the new stream.

Fixes #703 (closed)