Switch to netlink.RouteListFilteredIter() API

[fasaxc]

Description

Optimise RouteTable resync methods:

• Add benchmark for resync with various numbers of routes.

• Switch to netlink.RouteListFilteredIter() API, which avoids a series of large slice allocations.

• Use benchmark to find more wins:

  • Use deltatracker.WithValuesEqualFn, DeepEquals is the default and it was much slower.
  • Pre-allocate the seenKeys set, saves the expense of copying it multiple times as it grows.
  • Copy the netlink Route once per loop to a scratch variable to avoid escape to the heap.

Overall, significantly reduces allocations from when there are many routes to resync and significantly increases performance.

Benchmarks for resyncing 64k routes; 50% reduction in resync time; 80% saving in bytes allocated; GC-included CPU usage down by 60%.

v3.28:

BenchmarkResync65536-12    	       7	 156498055 ns/op	 242857143 ncpu/op	203556697 B/op	  790473 allocs/op

master:

BenchmarkResync65536-12    	       7	 143550698 ns/op	 270000000 ncpu/op	208147700 B/op	  724999 allocs/op

This PR: 

BenchmarkResync65536-12    	      13	  84051862 ns/op	 108461538 ncpu/op	36912350 B/op	  526080 allocs/op

Related issues/PRs

Follow on from #8979
CORE-10626

Todos

• Tests
• Documentation
• Release note

Release Note

Felix's route resync logic has been optimised; it now uses 50% less CPU time and 80% less memory.

Reminder for the reviewer

Make sure that this PR has the correct labels and milestone set.

Every PR needs one docs-* label.

• docs-pr-required: This change requires a change to the documentation that has not been completed yet.
• docs-completed: This change has all necessary documentation completed.
• docs-not-required: This change has no user-facing impact and requires no docs.

Every PR needs one release-note-* label.

• release-note-required: This PR has user-facing changes. Most PRs should have this label.
• release-note-not-required: This PR has no user-facing changes.

Other optional labels:

• cherry-pick-candidate: This PR should be cherry-picked to an earlier release. For bug fixes only.
• needs-operator-pr: This PR is related to install and requires a corresponding change to the operator.

[aaaaaaaalex]

Just one potential change, might be nothing

[fasaxc]

Just holding this back until I've got the RouteTable merge to Enterprise done. Might want to upstream some of the fixes there and I don't want this to conflict...