fix(ebpf): ignore kthreads, do full pids cleanup

[korniltsev]

This PR addresses two issues

1. address ebpf: pids map mem leak #2769

I've noticed I was incorrectly tracking kthreads, which I dont care about.

Ignore kthreads.

Also added a "stale check" to cleanup pids which are not existing, but we did not get a dead notification for them(I am not aware of any other reason, so in theory this should not be needed)

2. I've noticed sometime ebpf program requests information for the same PID multiple times

ts=2023-11-30T06:36:59.455026088Z level=debug msg="pid info request" component=pyroscope.ebpf.instance pid=1879972 target="{__container_id__=\"3a1ace6dfee59c11d984f688d8ef9a5c3a9dbf4c0be45ffdef15009f6631fb64\", __name__=\"process_cpu\", app=\"grafana\", conprof=\"true\", container=\"grafana\", insights=\"true\", name=\"grafana\", namespace=\"hosted-grafana\", node=\"gke-prod-us-central-0-hg-n2s8-9-b0d8ad12-gb65\", org=\"marathon\", plan=\"advanced\", pod=\"marathon-grafana-55cd8dfc94-8blkc\", pod_template_hash=\"55cd8dfc94\", service_name=\"ebpf/hosted-grafana/grafana\", slug=\"marathon\", stackId=\"492115\"}"
ts=2023-11-30T06:36:59.455026088Z level=debug msg="pid info request" component=pyroscope.ebpf.instance pid=1879972 target="{__container_id__=\"3a1ace6dfee59c11d984f688d8ef9a5c3a9dbf4c0be45ffdef15009f6631fb64\", __name__=\"process_cpu\", app=\"grafana\", conprof=\"true\", container=\"grafana\", insights=\"true\", name=\"grafana\", namespace=\"hosted-grafana\", node=\"gke-prod-us-central-0-hg-n2s8-9-b0d8ad12-gb65\", org=\"marathon\", plan=\"advanced\", pod=\"marathon-grafana-55cd8dfc94-8blkc\", pod_template_hash=\"55cd8dfc94\", service_name=\"ebpf/hosted-grafana/grafana\", slug=\"marathon\", stackId=\"492115\"}"

This can probably happen when multiple threads are profiled.
So i updated ebpf program to only request pid info once.

[cyriltovena]

Suggested change

	// it is only needed in case disassociate_ctty hook somehow mises a process death
	// it is only needed in case disassociate_ctty hook somehow misses a process death

[cyriltovena]

LGTM