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 EVENT_LOG_ERR 
# define EVENT_LOG_ERR _EVENT_LOG_ERR
#endif
    
    if (severity >= EVENT_LOG_ERR) {
        throw std::runtime_error(strprintf("libevent error: %s", msg));
    }
}
class CConnectionFailed : public std::runtime_error