Bug: Cache timeout under contention

[z64]

Using the demo code here:

https://paste.sr.ht/~z64/05e1209fea4e2d465b75e232720fba1ca72c96f8

Produces output:

$ cr _test.cr google.com 5
...................................................
  0 | 48.33ms
  1 | 48.34ms
  2 | 5024.24ms
  3 | 5024.25ms
  4 | 5024.25ms

because the domain is never removed from the pending list once it is resolved.

The following patch resolves this problem: https://git.sr.ht/~z64/durian.cr/commit/663ca66477d80c2ab8757d8f51669d6dd0e7e328

$ cr _test.cr google.com 5
.
  0 | 51.47ms
  1 | 51.48ms
  2 | 51.48ms
  3 | 51.48ms
  4 | 51.48ms

Please feel free to apply this patch or resolve at your discretion.