refator: sensor's architecture redesign

[iximiuz]

The redesign is supposed to make the codebase:

• More (unit-)test-friendly.
• More execution-friendly - all (but top-level) errutil.FailOn() were removed from the sensor logic.
• Prepared for Execution (not really a context) introduction.
• Prepared for incremental artifact storing.

Introduced abstractions:

• Sensor - to abstract differences in the standalone and controlled execution flows.
• Monitor (fanotify & ptrace) - makes an act of monitoring a noun instead of a verb (easier to pass around and interact with).
• CompositeMonitor - to abstract multiple monitors behind single entity and simplify the code of the (controlled and standalone) sensors.

Fixed bugs:

• IPC server startup racing with monitor startup - MonitorDoneEvent could have been sent before the EvtClient connection leading to sensor becoming stuck forver.
• Missed draining of the events channel in ptrace monitor when collection is done
  • files might have been randomly missed in the report.
• Fanotiry report partial initialization leading to panic when the monitored process exits relatively fast.

Discovered but not fixed bugs:

• When a file is deleted during tracing, it won't be included in the report (even though it should be). Two tests were written for this, but they are being skipped at the moment.

[ghost]

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Legend

[kcq]

can you also create an issue to track the not fixed discoveries

[iximiuz]

Here they go:

• 405

• 406