Reusable recipient registry

[xuhcc]

It should be possible to use recipient registry with multiple funding round factories, but currently SimpleRecipientRegistry allows only single controller.

• (Short-term solution) Do not call setMaxRecipients() from funding round factory. Trust that maxRecipients is set to correct value.
• Do not allow decreasing maxRecipients.
• Allow registry owner to call setMaxRecipients() (?).

There's an edge case where the number of recipients increases beyond MACI's vote option limit during the round (this can lead to users unknowingly voting for invalid vote options).
To prevent it the funding round factory should set maxRecipients at round deployment and lock it until the end of the round. If some other entity will set maxRecipients, it should similarly do that in sync with funding rounds (and with respect to vote option limit of currently deployed MACI instance).