Add worker pool to gather pid stat

[leborchuk]

Generate jobs for gather pid stat. Each job contain no more then JobsPerQuery pid's and belongs to a single host.

Jobs should be processed in a worker pool witch context, use for it alitto/pond - good enough to this work.

Here we add only worker pool without actual start working pool and doing some actual work.

Tests were generated. But I manually checked all of them - looks good.

Also refactor using statActivityLister - place links to it in background worker, no need to pass it as a parameter in every function.

[Copilot]

Pull request overview

Adds initial master-side infrastructure for collecting per-PID procfs stats by grouping session PIDs into per-host batches and executing requests via a worker pool, while also refactoring how session listing is injected into background components.

Changes:

• Introduces GatherProcfsStat/processProcfsRequests with host grouping and pond-based worker pool submission.
• Refactors BackgroundStorage to own a StatActivityLister dependency (and adjusts callers/tests accordingly).
• Extracts and reuses a shared gRPC client connection cache helper in internal/master/utils.go.

Reviewed changes

Copilot reviewed 13 out of 14 changed files in this pull request and generated 12 comments.

Show a summary per file

File	Description
internal/master/utils.go	Adds shared cached gRPC connection helper for master package.
internal/master/procfs.go	Implements job grouping + worker-pool execution for procfs PID stat RPCs.
internal/master/procfs_test.go	Adds unit tests for job grouping, request building, and pool behavior.
internal/master/deps.go	Extends statActivityLister with ListAllSessions.
internal/master/background.go	Moves session lister dependency into BackgroundStorage and updates refresh flow/InitBG wiring.
internal/grpc/testutils_test.go	Updates test server wiring to new BackgroundStorage + master info server signatures.
internal/grpc/set_get_query_info_parallel_test.go	Updates BackgroundStorage constructor usage and dialer setup.
internal/grpc/mocks_test.go	Extends test mock to satisfy updated lister usage in BackgroundStorage.
internal/grpc/get_master_info.go	Switches session refresh to use backgroundStorage’s lister; updates constructor signature.
internal/grpc/actions_test.go	Updates BackgroundStorage constructor call site.
internal/app/app.go	Wires real stat activity lister into BackgroundStorage; updates master info server construction and InitBG call.
go.mod / go.sum	Adds github.com/alitto/pond dependency.

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