Limit concurrency of .AddTablet() calls in discovery healthcheck

[timvaillancourt]

Description

This PR limits the number of concurrent .AddTablet(...) calls that can occur at the same time, to avoid hitting go runtime limits

A default of 1024 32 is used with a new flag named --tablet_discovery_concurrency --healthcheck_concurrency to limit this

To explain the go.sum update, CI requested it. I wonder if a previous merge had an inconsistent go mod setup:

go: downloading github.com/jtolds/gls v4.20.0+incompatible
go: downloading github.com/smartystreets/assertions v0.0.0-20190116191733-b6c0e53d7304
go: downloading github.com/gopherjs/gopherjs v0.0.0-20181103185306-d5[47](https://github.com/slackhq/vitess/actions/runs/7571374065/job/20618771953?pr=180#step:15:48)d1d9531e
go: downloading modernc.org/httpfs v1.0.6
go: downloading modernc.org/z v1.7.0
We wish to maintain a tidy state for go mod. Please run `go mod tidy` on your branch, commit and push again.
Running `go mod tidy` on this CI test yields with the following changes:
 M go.sum

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on the CI
• Documentation was added or is not required

Deployment Notes

[timvaillancourt]

Closing in favour of #199