[REST] getutxos REST command (based on Bip64)

has parts of @mhearn #4351
* allows querying the utxos over REST
* same binary input and outputs as mentioned in Bip64
* input format = output format
* various rpc/rest regtests

src/rpcserver.h

@@ -237,6 +237,7 @@ extern json_spirit::Value reconsiderblock(const json_spirit::Array& params, bool
 // in rest.cpp
 extern bool HTTPReq_REST(AcceptedConnection *conn,
                   const std::string& strURI,
+                  const std::string& strRequest,
                   const std::map<std::string, std::string>& mapHeaders,
                   bool fRun);