Keep track of active agent versions

This MR starts to keep track of agent versions that have active agents.

Based on the proposal, use two ExpiringHashs.

1. Agent Versions Hash:

Key	Hashkey	Value
agent_versions	version	ExpiringValue proto message with nil value field

2. Hash per version:

Key	Hashkey	Value
connections by agent version	ConnectionID	[]bytes of a protobuf message {agent_id:, pod_id:} wrapped in ExpiringValue

Part of #505 (closed).