mostro stats first release

[arkanoider]

Hi @grunch ,

here we go with the fixed pr.

Stats are basic, but it's a starting point. Now a file is saved every minutes with latest data collected. Values are updated inside mostrod at every event received, file is called mostro_stats.json with a simple struct like this:

pub struct MostroMessageStats {
    received: u64,
    new_order: u64,
    new_dispute: u64,
    release: u64,
    bytes_recv: u64,
    max_msg_bytes: u64,
    min_msg_bytes: u64,
    last_msg_bytes: u64,
}