gce: rm unnecessary GetMigInstanceTemplate locking

[bpineau]

The lock held from (and covering most of) GetMigInstanceTemplate() prevents
InstanceTemplates.Get() concurrent executions, while it's not really needed
as all caches access functions used here already take care of locking properly
(and it's reading/writing one single cache entry per call).

InstanceTemplates.Get() isn't fast (at about 0.5s/call in my test env),
running all calls sequentially slows down startup significantly:

forceRefresh()
  fetchAutoMigs()
    workqueue.ParallelizeUntil(..., func() { <- can launch concurrent registerMigs
      registerMig()
        GetMigTemplateNode()
          GetMigInstanceTemplate() <- lock held
            FetchMigTemplate()
              client.gceService.InstanceTemplates.Get() <- slow API call

When running this change on a cluster having 800 MIGs, startup went from 6min to 1.5min.
Also tested with a -race build.

Which component this PR applies to?

Cluster-autoscaler's GCE cloud provider.

What type of PR is this?

/kind cleanup
/kind feature

What this PR does / why we need it:

Remove unnecessary locking to speed-up startup time on large clusters (many MIGs).

[MaciekPytel]

@BigDarkClown can you take a look? I think you're the most familiar with this part of the code.

[BigDarkClown]

I think this change is reasonable. As you noticed the cache setters and getters are already guarded by a different lock, so there should be no issues with concurrency there.

In other MigInfoProvider functions the lock held here prevents excessive querying, as usually it takes only one a single call to fill the underlying cache. But for GetMigInstanceTemplate this is not really a case, for each MIG first call we will require a single API call either way. So removing this lock here seems like a good idea.

[BigDarkClown]

/lgtm

[k8s-ci-robot]

@BigDarkClown: changing LGTM is restricted to collaborators

In response to this:

/lgtm

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[bpineau]

Rebased + re-tested after the lock was renamed

[MaciekPytel]

Oops, it completely slipped my radar.
/lgtm
/approve
Sorry for delay :(

[k8s-ci-robot]

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This pull-request has been approved by: bpineau, MaciekPytel

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