Should serve the closest bands if low on tokens

[ghost]

If any of the market maker keepers is running out of SAI or WETH tokens, it should in the first order try to serve the bands closest to the reference price i.e. the ones with the lowest margin (spread).

I guess that currently this order is undetermined, it will probably depend on the ordering in which bands are specified in the config file.

[ghost]

Moved from: makerdao/pymaker#41