sweepbatcher: add mixed batches option

Option WithMixedBatch instructs sweepbatcher to create mixed batches with regard
to cooperativeness. Such a batch can include both sweeps signed both
cooperatively and non-cooperatively. If cooperative signing fails for a sweep,
transaction is updated to sign that sweep non-cooperatively and another round of
cooperative signing runs on the remaining sweeps. The remaining sweeps are
signed in non-cooperative (more expensive) way. If the whole procedure fails for
whatever reason, the batch is signed non-cooperatively (the fallback).

Author: starius