Mechanism to communicate 3PID binding updates or deletions to homeservers

[babolivier]

Documentation: https://docs.google.com/document/d/1LPHBfJQ6x5iOkm2_ZYPNHNjyAzJ4hWNdOGcPGpnEb14

[babolivier]

As @maxidor pointed out to me in PM, part of this proposal shoud be reworked to be compliant with spec HS 3.5 which states that bindings on ISs and bindings on HSs are independant, therefore we shouldn't consider changes regarding one to have direct consequences upon the other.

[uhoreg]

@babolivier Is this still needed? It seems like it's going in the opposite direction from the privacy work that was done last year.

[babolivier]

Looks like this has already been covered by the unbinding work that happened a while ago.