vtorc: cleanup discover queue, add concurrency flag

[timvaillancourt]

Description

This PR cleans up the VTOrc discover queue code, mainly by removing all locking from .Consume(); this was achieved by adding type queueItem struct and pushing this to the channel instead, with the key and time.Time included. Previously this time.Time was stored in a map with mutex locks

The .QueueLen() now returns just the len() of the queue channel. Before this metric would include the length of the channel plus all that are "in processing" (between .Consume() and .Release()), which I'm unsure is necessary. Simplifying this removed the need for a lock in .QueueLen()

Finally, the --discovery-max-concurrency flag was added to control how many workers consume the discovery queue

Benchmark:

tvaillancourt@tvailla-ltmawfe vitess % go test -v -bench=. ./go/vt/vtorc/discovery/...
=== RUN   TestQueue
--- PASS: TestQueue (0.00s)
goos: darwin
goarch: arm64
pkg: vitess.io/vitess/go/vt/vtorc/discovery
cpu: Apple M3 Max
BenchmarkQueues
BenchmarkQueues/New
BenchmarkQueues/New-14         	    4726	    244573 ns/op
BenchmarkQueues/Legacy
BenchmarkQueues/Legacy-14      	    3610	    338468 ns/op
PASS
ok  	vitess.io/vitess/go/vt/vtorc/discovery	3.923s

NOTE: New == this PR, Legacy == current queue

Related Issue(s)

#17330

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 77.41935% with 7 lines in your changes missing coverage. Please review.

Project coverage is 67.47%. Comparing base (0c6ad63) to head (45e87fd).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtorc/config/config.go	0.00%	4 Missing ⚠️
go/vt/vtorc/logic/vtorc.go	0.00%	2 Missing ⚠️
go/vt/vtorc/discovery/queue.go	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17825      +/-   ##
==========================================
+ Coverage   67.44%   67.47%   +0.03%     
==========================================
  Files        1592     1592              
  Lines      258076   258159      +83     
==========================================
+ Hits       174051   174201     +150     
+ Misses      84025    83958      -67     

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[timvaillancourt]

The previous code did return len(q.queue) + len(q.queuedKeys) and I'm not sure why 🤔

If we see a reason I can restore this but it adds a lock. I think only metrics call this lock