Dust sweeper feature implementation

[sjanel]

When user defines a threshold amount on a given currency, coincenter can be used to attempt to sell all amount below this threshold trying to reach exactly 0 amount on this currency for clean balance.
The algorithm used by coincenter is a simple heuristic:

• it finds the best markets involving currency to be sold
• it tries to sell all immediately.
• if full selling is not possible, it buys a small amount of this currency
• retry until either maximum number of steps (configurable) is reached or balance is exactly 0 on given currency