[HTTP endpoint] Introduce endpoint for providing node status

[boris-sinyapkin]

This is a major change that includes the following items:

• Endpoint responds to /healthcheck URI path with a) "started" UTC timestamp; b) "uptime" seconds. Data packed as JSON in HTTP response body.
• Server now has separate LurkServerStats instance for tracking of statistics
• HTTP endpoint handles requests asynchronously
• HTTP endpoint has atomic reference to server instance