adds shred-version to ip-echo-server response

When starting a validator, the node initially joins gossip with
shred_verison = 0, until it adopts the entrypoint's shred-version:
https://github.com/solana-labs/solana/blob/9b182f408/validator/src/main.rs#L417

Depending on the load on the entrypoint, this adopting entrypoint
shred-version through gossip sometimes becomes very slow, and causes
several problems in gossip because we have to partially support
shred_version == 0 which is a source of leaking crds values from one
cluster to another. e.g. see
https://github.com/solana-labs/solana/pull/17899
and the other linked issues there.

In order to remove shred_version == 0 from gossip, this commit adds
shred-version to ip-echo-server response. Once the entrypoints are
updated, on validator start-up, if --expected_shred_version is not
specified we will obtain shred-version from the entrypoint using
ip-echo-server.

validator/src/main.rs

@@ -2145,6 +2145,8 @@ pub fn main() {
         expected_bank_hash: matches
             .value_of("expected_bank_hash")
             .map(|s| Hash::from_str(s).unwrap()),
+        // TODO: Once entrypoints are updated to return shred-version, obtain
+        // cluster shred version from ip-addr-server if this is None.
         expected_shred_version: value_t!(matches, "expected_shred_version", u16).ok(),
         new_hard_forks: hardforks_of(&matches, "hard_forks"),
         rpc_config: JsonRpcConfig {