improve: support merging txt strings into one

some dns manager split long binary data into more than one strings (e.g. linode.com)

evdns_base_resolve_txt function is added by you? I didn't find it in original libevent source.

I think it's ok to try to merge txt strings.

You can test it with "--provider-name=2.dnscrypt-cert.yechengfu.com --provider-key=4298:5F65:C295:DFAE:2BFB:20AD:5C47:F565:78EB:2404:EF83:198C:85DB:68F1:3E33:E952" options.