Request peer status often and trigger sync after status

[ChihChengLiang]

What is wrong?

Per #1399 (comment)

First, Trinity only requests a peer's STATUS once happened at the handshake, so the peer pool never updates the information of the peer's head slot, finalized root, finalized epoch, etc.

Second, the spec says we should start the sync once we learn the STATUS, while it's not the case in the current implementation.

How can it be fixed

• We should request the peer's status once for a while to have the updated knowledge of the peer's finalized root and epoch.
• We should have a way to trigger sync immediately after the handshake.

[ralexstokes]

Closing due to staleness (and this has also been re-filed here: ethereum/trinity-eth2#17)