SnapshotHostInfo should expire when the epoch ends

[saketh-are]

SnapshotHostInfo advertises a snapshot made at the end of a specific epoch. The node will not retain the snapshot beyond the end of the following epoch.

Peers in the network store and share the most recent SnapshotHostInfo published by each node. Otherwise, there is no expiration logic. If a node leaves the network and stops publishing new snapshots, its old information will survive indefinitely.

Every time a new connection is made the entire set of SnapshotHostInfos are exchanged. Sharing old data wastes bandwidth, so we should add some expiry check to prevent it.