Compilation issue on `mipsel-unknown-linux-musl` target

[matszczygiel]

Hello,
I was recently compiling rtp crate for mipsel-unknown-linux-musl target and got compilation error about AtomicU64 not being defined.
As it turns out, this specific architecture does not support atomic operations on u64, so rust stdlib does not provide it.
Moreover during inspecton I noticed some synchronization problems with SequencerImpl::next_sequence_number. It is possible that
self.roll_over_count may accidentally be incremented by 2 when this method is called from multiple threads.
I will open a pull request which fixes that using simple Mutex,. It also fixes the compilation issue on mipsel target.

#[derive(Debug, Clone)]
struct SequencerImpl {
    sequence_number: Arc<AtomicU16>,
    roll_over_count: Arc<AtomicU64>,
}

impl Sequencer for SequencerImpl {
    /// NextSequenceNumber increment and returns a new sequence number for
    /// building RTP packets
    fn next_sequence_number(&self) -> u16 {
        let sequence_number = self.sequence_number.load(Ordering::SeqCst);
        if sequence_number == u16::MAX {
            self.roll_over_count.fetch_add(1, Ordering::SeqCst);
            self.sequence_number.store(0, Ordering::SeqCst);
            0
        } else {
            self.sequence_number
                .store(sequence_number + 1, Ordering::SeqCst);
            sequence_number + 1
        }
    }

    /// RollOverCount returns the amount of times the 16bit sequence number
    /// has wrapped
    fn roll_over_count(&self) -> u64 {
        self.roll_over_count.load(Ordering::SeqCst)
    }

    fn clone_to(&self) -> Box<dyn Sequencer + Send + Sync> {
        Box::new(self.clone())
    }
}