zb: Disable overflow on Connection's broadcast channel & high-level API rework
This PR address a bunch of issues with some fundamental API and internal changes:
- Fixes #178 (closed) by disabling overflow-mode on the main broadcast channel.
- Fixes #188 (closed) by dropping
Proxy::next_signal
in favor of an internal task that receives and dispatches incoming signal messages for the user. - Fixes #195 (closed) by dropping
ObjectServer::try_handle_next
(and removingObjectServer::dispatch_message
from pub API) in favor or an internal task the receives and dispatches incoming method calls for the user. This also creates a 1-1 association between a Connection and ObjectServer.