Split global mutex into granular locks

[mark-e-deyoung]

Problem

ServerState::mu is a single std::mutex protecting 8 data structures. Under 32 concurrent snapshot.capture requests, one lock serializes all allocation.

Proposed Solution

Split into three locks:

• snapshots_mu: snaps + pinned_counts + lru_order + snap_counter
• sessions_mu: sessions + sessionCount
• event_mu: event_counter + event_log

No operation holds more than one lock currently, so this is a safe split.

References

• Scalability audit item 4