Remove support for SRV records.

[lambdafu]

This is based off #24 and removes support for SRV DNS records. SRV records are used to advertise alternative services in a domain are available, such as LDAP, XMPP or SIP. The SKS Keyserver HKP Pools use them to indicate the quality of a keyserver that is then accessed via http/https by their weights.

I removed support for SRV DNS records from NeoPG:

• They are not supported by PGP.
• Although interesting, the use of SRV weights to optimize the keyserver pool access is highly unorthodox for HTTP/HTTPS load balancing. I understand it is an attempt to eliminate the middle-man, but it is not clear what the threat model here is that motivates these complexities.
• It is not supported by libcurl although it has been proposed 2006 specifically to support the use case in OpenPGP.
• I know of no other keyserver pool doing this (or well, any other keyserver pool for that matter). It's an island.

If there ever will be curl support for it, we might reconsider this.

[lambdafu]

This feature, along with #24, also made it necessary to implement a special tor mode instead just relying on the socks5h proxy.