Networking and syncing tweaks (#2990)

* Reduce periodic snapshot delay

* Prevent connecting to peers over the min_peers limit

* Smaller block requests for slower connections

parity/run.rs

@@ -53,7 +53,7 @@ use url;
 const SNAPSHOT_PERIOD: u64 = 10000;
 
 // how many blocks to wait before starting a periodic snapshot.
-const SNAPSHOT_HISTORY: u64 = 500;
+const SNAPSHOT_HISTORY: u64 = 100;
 
 #[derive(Debug, PartialEq)]
 pub struct RunCmd {