server: lookup DNS seeds directly by active chain's genesis hash
server: lookup DNS seeds directly by active chain's genesis hash

In this commit, we fix a bug that would cause the DNS seeds to be
*always* active regardless of which chain+network we were on. Before we
would look up the network in the reverseChainMap. However, if we were
on regtest or testate, then it would still (incorrectly) resolve to a
valid hash.

To remedy this, we now directly use the genesis hash of the current
active chain.

aakselrod/lnd,LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd