sweepbatcher: add mode with presigned txs and CPFP

In this mode sweepbatcher uses transactions provided by the CPFP helper, which
may be pre-signed and not replace-by-fee (RBF) compatible. In such cases, CPFP
may be necessary.

A single Batcher instance can handle both CPFP and regular batches. Currently
CPFP and non-CPFP sweeps never appear in the same batch.

Author: starius