Rewrite DNS response cache

[0xERR0R]

Current implementation has some drawbacks:

• it uses https://github.com/0xERR0R/go-cache -> which is a fork of not maintained project with some merged PRs
  • LRU implementation is not optimal
• workaround with TTL (see Caching doesn't seem to work properly #290): there is a gap if some entry is expired, but not removed -> blocky returns response with TTL 0

New implementation should be created:

• Cache with limited element size (max elements)
• LRU
• TTL support
• callback function before entry is expired, which allows to refresh the entry and set new TTL

[kwitsch]

Bench tested 0.17 vs 0.18 resolution time of cached DNS entries.

Results:
0.17: ~15ms
0.18: ~6ms

For testing purpose i deployed the different versions on the same docker host with and identical configuration.
Each version was tested with DNS-Bench & DNS Test(Android App).

I'm positively impressed.
Great job. 👍

[0xERR0R]

Hi, that is cool. My goal was to remove the forked implementation of not maintained project. If the new implementation is also faster - this is a double win👍