Add ability for sync::watch::Receiver to flag current item as unseen

[allada]

Is your feature request related to a problem? Please describe.
Currently there is no way for a sync::watch::Receiver to be created and mark the current item as unseen without notifying all Receivers.

Describe the solution you'd like
Add a function on Receiver that allows the next time the Receiver calls .has_changed() (or similar) it returns true.

Describe alternatives you've considered
Add similar function to Sender::subscribe() where newly created Receiver has a message pending and Receiver::clone()-like method that does the same thing.

Use case

• Client connects with some request.
• These requests can be multiplexed, so only 1 unique request is ever being processed, but clients all get notified of state changes (only most recent state matters).
• When first client connects they are notified of a change 100% of the time because sync::watch::channel() always creates a Receiver with a pending message.
• When a new client connects, we want to notify the client of the current state.

This is a very minor security concern for us because unless we hold a copy of the last seen data for every Receiver we are forced to re-send a bogus state change to every Receiver, which can then be used by the receiver client to get hints on when people are connecting and disconnecting (yes, this could be solved on a higher level, but requires a lot of needless work & memory).

[victor-timofei]

If no one is working on this I'd like to pick this up.

[Darksonn]

Go ahead.