api2/node/dns.rs: implement concurrent update protection

src/tools/common_regex.rs

@@ -34,4 +34,6 @@ macro_rules! IPRE { () => (concat!(r"(?:", IPV4RE!(), "|", IPV6RE!(), ")")) }
 
 lazy_static! {
     pub static ref IP_REGEX: Regex = Regex::new(IPRE!()).unwrap();
+
+    pub static ref SHA256_HEX_REGEX: Regex = Regex::new("^[a-f0-9]{64}$").unwrap();
 }