Support multiple Hydra Heads on-chain

[ch1bo]

What & Why

Current hydra-node implementation doesn't distinguish between on-chain Hydra Head protocol transactions belonging to multiple open Hydra Heads in the network at the same time. As a consequence, only a single Hydra Head is supported at any time.

This is only acceptable in a private devnet environments, and needs to be implemented to run hydra-node in a Cardano public testnet environment.

Furthermore, hydra-node needs to use provided --network-magic consistently throughout the node and we aim to add / update documentation on how to launch everything using the public Cardano testnet within this scope.

TBD

This is related to #181 and maybe we even need state thread tokens to implement this!