libevent seems to have invaded `libbitcoin_common` via `url.cpp`

`url.cpp` is used only from `test/fuzz/string.cpp` and `wallet/rpcwallet.cpp`

libevent should only be required for bitcoind, bitcoin-cli and (due only to the hack of passing wallet names) bitcoin-qt

#ifndef BITCOIN_UTIL_URL_H
#define BITCOIN_UTIL_URL_H
#include <string>
std::string urlDecode(const std::string &urlEncoded);
#endif