sds: support (de)serialization of a message channel

[adklempner]

In the browser context, state is often cleared due to refreshes or disconnection. Creating a message channel from scratch each time can lead to lost messages and unnecessary queries.

We likely want to make it easy to store and recover data related to a message channel in local storage. This requires functions to serialize and deserialize the state of a message channel, and a .from function in addition to the regular constructor.

At minimum, it should be possible to create an instance of MessageChannel given a bloom filter and local history.