Different priority levels for system segments vs external segments.

[andreipaduroiu]

Problem description
System segments must bypass throttling checks and any operations on them should be executed before external operations.

This should ease up self-induced cache pressure and speed up LTS transfers for SLTS implementations.

Problem location
Segment Store/Ingestion Pipeline.

Suggestions for an improvement
Implement basic QoS. Define at least 2 priorities (Normal and System). System is assigned to critical, internal segments, everyone else gets Normal. Critical segments are not subject to throttling and, outside of that, would be processed ahead of other operations.