prepare for migration to zcf atomicTransfer()

[Chris-Hibbert]

@erights came up with a new approach to managing reallocation in contracts (6577) that we have been very happy with. We have been planning to migrate the temporary helper into ZCF (6679) and remove the old approach, but we neglected to warn the developers that this major change was coming.

We should, ASAP, prepare documentation on use of the atomicTransfer helper, and explain that it will be moved into ZCF shortly, but more importantly, we should add deprecation annotations to the existing incrementBy and decrementBy ZCF functions so our developers know what to expect.

Luckily, all our contracts are now written in the interim style, so anyone reading our code will see examples of the new approach.

[Chris-Hibbert]

zcfSeat.getStagedAllocation() and zcfSeat.hasStagedAllocation() should also be deprecated. They'll go away when we remove the old approach, and aren't needed while using the interim approach.

[Chris-Hibbert]

We did deprecate the methods at their declarations, so it's better than I said. Developers (at least those using IDEs) would have seen the deprecation warning if they tried to use the old methods.

It's still a problem that we haven't documented the new approach or updated the documentation to show the deprecation.