network: prevent useless getroots requests
At the height of A we only reliably have a state root for A-1, so there is no
point in requesting state root for A from our peers. Prevents tons of useless
traffic:
2020-08-10T00:02:35.537+0300 DEBUG got msg {"addr": "127.0.0.1:20333", "type": "getroots"}
2020-08-10T00:02:35.537+0300 DEBUG got msg {"addr": "127.0.0.1:20333", "type": "roots"}
2020-08-10T00:02:35.537+0300 DEBUG got msg {"addr": "127.0.0.1:20333", "type": "getroots"}
2020-08-10T00:02:35.537+0300 DEBUG got msg {"addr": "127.0.0.1:20333", "type": "roots"}
2020-08-10T00:02:35.539+0300 DEBUG got msg {"addr": "127.0.0.1:20335", "type": "getroots"}
2020-08-10T00:02:35.539+0300 DEBUG got msg {"addr": "127.0.0.1:20335", "type": "roots"}
2020-08-10T00:02:35.539+0300 DEBUG got msg {"addr": "127.0.0.1:20334", "type": "getroots"}
2020-08-10T00:02:35.539+0300 DEBUG got msg {"addr": "127.0.0.1:20334", "type": "roots"}
2020-08-10T00:02:35.540+0300 DEBUG got msg {"addr": "127.0.0.1:20334", "type": "getroots"}
2020-08-10T00:02:35.540+0300 DEBUG got msg {"addr": "127.0.0.1:20334", "type": "roots"}
2020-08-10T00:02:35.542+0300 DEBUG got msg {"addr": "127.0.0.1:20336", "type": "getroots"}
2020-08-10T00:02:35.542+0300 DEBUG got msg {"addr": "127.0.0.1:20336", "type": "roots"}