sweep: add time-based sweeper

In this commit, the sweep package is extended from just tx generation to
an active sweeper that collects sweep inputs and autonomously proceeds
to publish the sweep tx after the batch window time interval has passed
without new inputs being added.

sweep/defaults.go

@@ -0,0 +1,14 @@
+// +build !rpctest
+
+package sweep
+
+import (
+	"time"
+)
+
+var (
+	// DefaultBatchWindowDuration specifies duration of the sweep batch
+	// window. The sweep is held back during the batch window to allow more
+	// inputs to be added and thereby lower the fee per input.
+	DefaultBatchWindowDuration = 30 * time.Second
+)