Implement escrow model for sell orders

[ryanchristo]

Summary

For now, we have decided to implement an escrow model for sell orders. At a future date, we might want to consider an escrow module for buy orders but this is out-of-scope for this issue.

We will also need to add a field to the balance in the proto files for "escrowed balance" or "in sell orders". The total balance will include the escrow balance and the tradable balance.