SUBSCRIBE does not need a forward parameter if we have filters

[wilaw]

Currently within SUBSCRIBE you can set a forward parameter of 0 (don't send me anything) or 1 ( send me everything, default).

If we implement FILTERS (see #1164 and also Filter design team results), then we can remove this flag as:

a forward flag of 0 is syntactically equivalent to a FILTER that blocks everything
a forward flag of 1 is syntactically equivalent to a FILTER that allows everything

Forward state can be managed by SUBSCRIBE_UPDATE and adjusting the FILTER.

[afrind]

Parking until design team resolves.