nim's CI bottlenecked by https://github.com/status-im/nim-chronos
the slowest pipeline in nim CI is now Windows_amd64_pkg, and the slowest test in Windows_amd64_pkg is https://github.com/status-im/nim-chronos; 
https://github.com/status-im/ tests combine to 477s = 8mn, or 25% of total in PackageFileParsed;

```nim
2020-03-01T03:42:14.7562549Z PASS: https://github.com/status-im/nim-chronos C                   (266.27445602
2020-03-01T03:42:14.7559002Z PASS: https://github.com/status-im/nim-bncurve C                   (141.18012929
2020-03-01T03:42:14.7558235Z PASS: https://github.com/status-im/nim-blscurve C                  (48.72544289
2020-03-01T03:42:14.7561947Z PASS: https://github.com/status-im/nim-chronicles C                (19.57475209
2020-03-01T03:51:16.0747805Z PASS: https://github.com/status-im/nim-stint C                     ( 3.05449986
```

how about introducing a switch to make some of these tests (in particular https://github.com/status-im/nim-chronos and https://github.com/status-im/nim-bncurve) faster and replacing important_packages.nim by:
```nim
pkg "chronos", "", true
=>
pkg "chronos", "nimble testCI", true # or testFast
```
with something like:
```nim
task testCI, "test for nim CI":
  nim c -r -d:chronosFastCI` test.nim
```
where defined(chronosFastCI) can be used in the slowest tests to reduce number of iterations or similar logic
