Allow an address to receive more than one DataCap assignment

[jnthnvctr]

Client's can currently receive DataCap from a Notary at a given address. After spending that DataCap, if a Client wants more they have to provide the Notary with a new address in order to receive a new allocation of DataCap.

This introduces complexity for apps / users that require "top-ups" of DataCap. As an example - for an app like Slate they would need to behind the scenes monitor DataCap and usage for users (and swap out the addresses receiving and storing DataCap).

EDIT(@jennijuju)

Currently, datacap is assigned by notaries by using ./lotus-shed verifreg, which should be change to the ./lotus-shed notary terminalogy and also the cmd should be moving to a lotus cmd.

Instead of making a client verified client and then send a one-time upfront allocation, simply do a ./lotus notary send-datacap to allocate DataCap to a client account.

blocked by filecoin-project/specs-actors#1325

[jennijuju]

Reopen the ticket when FIP 0012 is accepted and implemented in spec actor

[jennijuju]

Reopening the issue as fip is now accepted and the actor implementation is ready filecoin-project/specs-actors#1390

[jennijuju]

Blocked by getting v4 actor dependency.

[jennijuju]

blocked by #6022

[BigLep]

Remaining work:

1. Unit test in Lotus
2. Get UX feedback from Product people and do anything requested (Refactor verifreg CMD #6194)

We can't close this until actors v5 is in master.