MEDIUM: resolvers: add supports of TCP nameservers in resolvers.

MEDIUM: resolvers: add supports of TCP nameservers in resolvers.

This patch introduce the new line "server" to set a TCP
nameserver in a "resolvers" section:

server <name> <address> [param*]
  Used to configure a DNS TCP or stream server. This supports for all
  "server" parameters found in 5.2 paragraph. Some of these parameters
  are irrelevant for DNS resolving. Note: currently 4 queries are pipelined
  on the same connections. A batch of idle connections are removed every
  5 seconds. "maxconn" can be configured to limit the amount of those
  concurrent connections and TLS should also usable if the server supports
. The current implementation limits to 4 pipelined

The name of the line in configuration is open to discussion
and could be changed before the next release.