Add mocking to test network/cache behavior

[phillip-stephens]

Our current mocking for unit tests is at the highest level of abstraction (DoSingleDstServerLookup) which avoids testing a lot of inner logic. After seeing #449, I think we need a way to ensure that some guarantees about how ZDNS is supposed to operate hold true.

For example:

• TCP/TLS/HTTPS sessions should be re-used
  • ./zdns A google.com yahoo.com microsoft.com apple.com eBay.com zdns-testing.com Cloudflare.com--threads=2 --tcp-only should result in only 2 TCP handshakes taking place
• GTLD servers should only be queried once from the roots per-gTLD
  • google.com and yahoo.com both have the same gTLD. In ./zdns A google.com yahoo.com --threads=1 --iterative, ZDNS should only make 1 query to the set of root IP addresses
  • This can be extended to include 2 sub-domains of google.com, and we should only query the .com name servers once for google.com's nameservers

Talked with @zakird and thinking the best approach is to introduce some sort of mocking for reliable tests.