watcher: provide interface(s) for sanitizing L1 state

[pgkeller]

Currently watcher is hard coded to check the state that it knows about (eg, L1[0], launch message, etc).

A more general approach would be to provide an interface such that the mechanism is separated from the policy. Internal watcher would loop over registered requests and call them for validation.

To keep the number of watcher state reads manageable, this should likely be done as 2 pieces of state:

• core/address/len to read
• N callbacks to validate the state
  (better than N reads over the NOC to each core)

This mechanism will enable, eg, fabric to manage its own state.

[pgkeller]

fyi @SeanNijjar