note: flag to stop synching

[baabeetaa]

atm, old subnodes have to set seeds and peers to empty to stop synching.
It works, however the node do not join the p2p network so that its not possible to provide blockchain data for other nodes to sync.

Figure out is there a better way to make the node stop synching but still able to join the p2p network.
Could be a patch in tendermint or cosmos-sdk or chain.