pool: connect relays on-demand

[yukibtc]

Add option to connect relays on demand (disabled by default). This can be useful for gossip, when (most likely) many relays are added to the pool.
If the option is enabled, automatically put on "sleep" (disconnect) the relays after some time and reconnect them on-demand (i.e., when sending an event, when fetching, ...). If there are active long-lived subscriptions, keep the connection up.

Alternatively, put on "sleep" relays that have only the GOSSIP flag and that have no active long-lived subscriptions.