Port 80 features from server/events.go including the full events infrastructure:
internal send/receive loops, system subscription machinery, statsz heartbeats,
remote server tracking, connection event advisories, user-info handler, OCSP peer
reject events, remote latency merge, kick/ldm client, and helper functions.
Add ClearConnectionHeartbeatTimer/SetConnectionHeartbeatTimer to Account,
add MsgHandler/SysMsgHandler delegates and supporting types (ServerApiResponse,
EventFilterOptions, StatszEventOptions, UserInfo, KickClientReq, LdmClientReq,
AccNumSubsReq) to EventTypes.cs, and add Seq field to ServerInfo for heartbeat
sequence tracking.
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