refactor: Wallet owns its update stream

[tvolk131]

Previously, calling Wallet::get_update_stream() spawned a new task on every call. Now, instead, the Wallet owns a single task that polls the federation views, and get_update_stream() simply creates a new stream that watches for the views that are produced by that single task. This also allowed us to add Wallet::force_update_view() which tells the Wallet-owned task to immediately produce the next value rather than waiting, and waits for the latest value to be produced. This ensures that force_update_view() doesn't complete until all streams returned by get_update_stream() have the latest view queued up. We then call force_update_view() at the end of all public Wallet functions that mutate client state. This ensures that the GUI received the latest view from the Wallet view stream before an action function completes.