Add a MIN_VERSION constant to zebra_network.

When we perform a handshake with a remote peer, we need to encode the
version messages with a particular network version before we find out
what the remote peer's version preference is.  So in addition to having
a CURRENT_VERSION constant (which represents our preference), we need to
have a MIN_VERSION during the handshake (and later to determine whether
we'll talk to the peer at all).

zebra-network/src/constants.rs

@@ -8,6 +8,9 @@ pub const USER_AGENT: &'static str = "Zebra v2.0.0-alpha.0";
 /// The Zcash network protocol version used on mainnet.
 pub const CURRENT_VERSION: Version = Version(170_007);
 
+/// The minimum version supported for peer connections.
+pub const MIN_VERSION: Version = Version(170_007);
+
 /// Magic numbers used to identify different Zcash networks.
 pub mod magics {
     use super::*;