TracerouteEngineImpl: limit number of traces materialized from TraceDag.

[anothermattbrown]

TraceDags have been observed to contain huge numbers of traces, too many to materialize into a list. Limiting to 10k for now. Rewrote TraceDagImpl to use iterators internally, as limit() wasn't working on the previous implementation based on recursive streams flattened with flatMap. Something about the stream implementation materializes many more traces than the limit, which wastes compute time and risks OOM. Rewriting using the same style of code but based on iterators solved the problem.

[batfish-bot]

This change is 

[codecov]

Codecov Report

Merging #8939 (12b010f) into master (ec8fb08) will decrease coverage by 0.01%.
Report is 1 commits behind head on master.
The diff coverage is 86.66%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8939      +/-   ##
==========================================
- Coverage   72.53%   72.52%   -0.01%     
==========================================
  Files        3321     3323       +2     
  Lines      169579   169605      +26     
  Branches    19912    19914       +2     
==========================================
+ Hits       123003   123009       +6     
- Misses      37415    37432      +17     
- Partials     9161     9164       +3     

Files	Coverage Δ
...n/java/org/batfish/common/traceroute/TraceDag.java	100.00% <100.00%> (ø)
...va/org/batfish/common/traceroute/TraceDagImpl.java	87.30% <100.00%> (+0.86%)	⬆️
...va/org/batfish/dataplane/TracerouteEngineImpl.java	96.87% <100.00%> (+0.32%)	⬆️
.../java/org/batfish/common/util/FlatMapIterator.java	77.77% <77.77%> (ø)

... and 4 files with indirect coverage changes

[dhalperi]

Reviewed 4 of 4 files at r1, 1 of 1 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @anothermattbrown)

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projects/batfish/src/main/java/org/batfish/dataplane/TracerouteEngineImpl.java line 69 at r1 (raw file):

                        .getValue()
                        .getTraces()
                        .limit(10000)

would probably 1/ extract to a constant in the interface 2/ add a limited variant in the interface where this const is the default 3/ migrate interface default impl to be the limited version of the constant.

Code quote:

10000

[dhalperi]

Reviewed 3 of 3 files at r3, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @anothermattbrown)