Reduce number of concurrent DNS lookups in Machine Tracker

[stveit]

Fixes #2669

Limits concurrent parallel lookups to 100 at a time.

Tested manually that machine tracker no longer crashes from hitting the file descriptor limit with ulimit set as low as 200 on a search with 4096 results. This would previously crash instantly

The amount of parallel lookups should perhaps be configurable but that can be separate pr if we see the need

A practical change I made is that saving the results from a lookup to the dict self.results will now happen during the parallel lookups, instead of at the end. I made this change simply because using Cooperator changes how the DefferedList we use look like. Instead of all the separate lookup tasks being in the list, the list now contains 100 iterators. I dont know how to add a callback to the list that will actually be able to go through all the results produced instead of just going over the iterators themselves, so I moved the saving step to be part of the smaller work units.

Errors that were previously raised by themselves are now eaten up by the Cooperator. Tried to fix this by saving all errors then raising the first one that occurred. I expect this keeps the interfacing with the asyncdns module intact, but side effect is that it will only raise the exceptions after its done running, so its slower to fail

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.83333% with 1 line in your changes missing coverage. Please review.

Project coverage is 59.52%. Comparing base (a2be786) to head (93fca87). Report is 87 commits behind head on 5.10.x.

Files	Patch %	Lines
python/nav/asyncdns.py	95.83%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 5.10.x #2917 +/- ## ========================================== - Coverage 60.21% 59.52% -0.69% ========================================== Files 601 602 +1 Lines 43981 43308 -673 ========================================== - Hits 26481 25780 -701 - Misses 17500 17528 +28 ```

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Test results

     11 files       11 suites   11m 8s :stopwatch: 3 325 tests 3 325 :heavy_check_mark: 0 :zzz: 0 :x: 9 443 runs  9 443 :heavy_check_mark: 0 :zzz: 0 :x:

Results for commit 93fca874.

:recycle: This comment has been updated with latest results.