Retrieval Should Not Block on Chain Operations

[hannahhoward]

As a client or provider, I would like modification operations with the chain to return immediately, and deliver their results asynchronously

---

In:
The only chain modifying function in the retrieval market is GetOrCreatePaymentChannel -- and it's made complicated by the fact that it may create a channel or it may add funds to a channel -- both operations create a chain message, but one that is interpreted differently (create channel contains the address in the message receipt, where add funds just has an exit code)

The current signature is as follows:

GetOrCreatePaymentChannel(ctx context.Context, clientAddress address.Address, minerAddress address.Address, clientFundsAvailable abi.TokenAmount, tok shared.TipSetToken) (address.Address, error)

The new signature should be

GetOrCreatePaymentChannel(ctx context.Context, clientAddress address.Address, minerAddress address.Address, clientFundsAvailable abi.TokenAmount, tok shared.TipSetToken) (address.Address, cid.Cid, error)

Where cid is the message cid to wait for

If the returned address == address.Undef that means a payment channel had to be created. If returned address is not address.Undef, that means a payment channel exists, but we're waiting to add funds.

the retrieval client should transition in to one of two states (based on whether returned address = address.Undef):
DealStatusPaymentChannelCreating
DealStatusPaymentChannelAddingFunds

each state will wait on one of two new APIS:

WaitForPaymentChannelAddFudns(messageCID cid.CId) error
WaitForPaymentChannelCreation(messageCID cid.Cid) (address.Address, error)

DealStatusPaymentChannelCreated can be renamed DealStatusPaymentChannelReady since that makes more sense

Assuming the states succeed, they transition to DealStatusPaymentChannelReady and the deal proceeds.