Allow changing packet sharing behavior in old p2p networking via ini config

[otavepto]

• By default, P2P manager will share packets flagged "unreliable", otherwise packets won't be shared (hybrid mode)
• Allow specifying the packet sharing mode for the old P2P networking interface ISteamNetworking via ini config
• Remove channels with no packets in P2P manager
• Remove outdated packets in P2P manager, packets existing for more than 20 seconds will be removed to avoid increasing memory consumption forever in case nothing was consumed over time

I noticed that the older games which require sharing, send packets with flag k_EP2PSendUnreliable or k_EP2PSendUnreliableNoDelay.
While the other games which require the opposite use k_EP2PSendReliable.
So I assumed this is the differentiating factor, but since that's just a wild guess, I added an option in the ini file to allow switching between the hybrid, old, and new behaviors.

Fixes and closes #385