a8n: Set an upper time limit on CampaignPlan execution

Created by: mrnugget

Currently a CampaignPlan and its CampaignJobs can run indefinitely in case the user specifies wait = false in the previewCampaignPlan mutation:

https://github.com/sourcegraph/sourcegraph/blob/034cee47c83a7734cf04a4b5720717665b5a69db/enterprise/pkg/a8n/resolvers/resolver.go#L445-L451

But we probably don't want single jobs to run for more than, let's say, 2 minutes.

So when we run a single CampaignJob we want to use a timeout context:

https://github.com/sourcegraph/sourcegraph/blob/034cee47c83a7734cf04a4b5720717665b5a69db/enterprise/pkg/a8n/run/runner.go#L152

If a CampaignJob runs into a timeout, we need to persist that error on the CampaignJob.

If the user specified wait = true a request timeout kicks in after 60s. We need to confirm that this timeout propagates down to the jobs and also results in an error.

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We also want to make sure that we use a correct timeout when talking to the replacer service in the comby CampaignType:

https://github.com/sourcegraph/sourcegraph/blob/034cee47c83a7734cf04a4b5720717665b5a69db/enterprise/pkg/a8n/run/campaign_type.go#L92-L111

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And maybe we should use a different fetchTimeout when talking to replacer, depending on whether or not the 500ms default value makes sense or not:

https://github.com/sourcegraph/sourcegraph/blob/d56ce18d1aa571fb5665931383c5fc9bc691223e/cmd/replacer/replace/replace.go#L180-L186