* Initial commit of signal deduper.
Idea is protect against signal storm from misbehaving clients.
Design:
- SignalDeduper interface with one method to handle a SignalRequest and
return if dupe or not.
- Signal specific deduper. Could have made a single de-duper which could
handle all signal message types, but making it per type so that the
code is cleaner.
- Some module (like the router) can instantiate whatever signal types
it wants to de-dupe. When a signal message is received, that module
can run the signal message through the list of de-dupers and
potentially drop the message if any of the de-dupers declare that the
message is a dupe. Making it a list makes things a little bit
inefficient, but keeps things cleaner. Hopefully, not many de-dupers
will be needed so that the inefficiency is not pronounced.
* re-arrange comments
* helper function
* add ParticipantClosed