sdk/agent: make snapshot files contain everything

[leighmcculloch]

In #227 (#273) we added snapshotting where the agent snapshots its internal state and that can be persisted however the application chooses.

The way snapshotting works right now is the snapshot only contains the internal state of the channel and not any of the config that was provided to the channel/agent at initialization. The reason that other information doesn't live in the snapshot is so that for the channel/agent there is only a single location that each piece of data is sourced from, and so that the SDK doesn't have to deal with what if there's a divergence between the two.

@acharb pointed out that it might not be well understood by a developer using the SDK that the snapshot doesn't contain that other information.

We should revisit this and see if there's an elegant way to keep the info all in the snapshot.