Report server domain to other servers:

This commit introduces a new configuration option that server
operators can set. The value is communicated to other servers
and is also reported via the `server_info` API.

The value is meant to allow third-party applications or tools
to group servers together. For example, a tool that visualizes
the network's topology can group servers together.

Similar to the "Domain" field in validator manifests, an operator
can claim any domain. Prior to relying on the value returned, the
domain should be verified by retrieving the xrp-ledger.toml file
from the domain and looking for the server's public key in the
`nodes` array.

src/ripple/app/misc/impl/Manifest.cpp

@@ -90,30 +90,9 @@ deserializeManifest(Slice s)
         {
             auto const d = st.getFieldVL(sfDomain);
 
-            // The domain must be between 4 and 128 characters long
-            if (boost::algorithm::clamp(d.size(), 4, 128) != d.size())
-                return boost::none;
-
             m.domain.assign(reinterpret_cast<char const*>(d.data()), d.size());
 
-            // This regular expression should do a decent job of weeding out
-            // obviously wrong domain names but it isn't perfect. It does not
-            // really support IDNs. If this turns out to be an issue, a more
-            // thorough regex can be used or this check can just be removed.
-            static boost::regex const re(
-                "^"                   // Beginning of line
-                "("                   // Beginning of a segment
-                "(?!-)"               //  - must not begin with '-'
-                "[a-zA-Z0-9-]{1,63}"  //  - only alphanumeric and '-'
-                "(?<!-)"              //  - must not end with '-'
-                "\\."                 // segment separator
-                ")+"                  // 1 or more segments
-                "[A-Za-z]{2,63}"      // TLD
-                "$"                   // End of line
-                ,
-                boost::regex_constants::optimize);
-
-            if (!boost::regex_match(m.domain, re))
+            if (!isProperlyFormedTomlDomain(m.domain))
                 return boost::none;
         }