sweepbatcher: add mode with presigned transactions

In this mode sweepbatcher uses transactions provided by the Presigned helper.
Transactions are signed upon adding an input to a batch.

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

Author: starius

sweepbatcher/log.go

@@ -50,8 +50,3 @@ func infof(format string, params ...interface{}) {
 func warnf(format string, params ...interface{}) {
 	log().Warnf(format, params...)
 }
-
-// errorf logs a message with level ERROR.
-func errorf(format string, params ...interface{}) {
-	log().Errorf(format, params...)
-}