Add escrow_accept operation

[theoreticalbts]

Related: #143

Escrow needs an additional step where from, to and agent all agree to participate. For example, if A wants to send to B using C as escrow, but C doesn't want to do business with A, C has no way to indicate this to the parties before the fee is given to C. Likewise, if B only wants to participate if C agrees to act as escrow agent [1], then C and B have to work out some confirmation protocol where B doesn't send the goods until C signs off on the request, essentially verifying off-chain that C is willing to be the escrow agent for this transaction.

If we had a working multisig UI, we would be able to simply require A, B, and C to sign the escrow_transfer_operation. But we don't, so we need another object to accumulate partial signatures.

• Replace escrow_transfer with propose_escrow and accept_escrow, where propose_escrow creates the object and accept_escrow adds signatures
• propose_escrow has expiration field
• propose_escrow has a proposer field which must be one of from, to or agent
• accept_escrow must be called to add sigs for other two parties
• When from does propose_escrow or accept_escrow, funds move from from balance to escrow object
• Fee is not given to agent until all parties have signed
• If object created by propose_escrow expires, funds return to from, and agent does not receive fee
• escrow_dispute and escrow_release work as before

[1] This is kind of the point of escrow -- if A/B didn't care about whether C was going to do their job, they wouldn't use escrow in the first place!

[mvandeberg]

Escrow has been merged for some time.