sweep: create wallet interface

We need access to additional wallet functionality. This commit creates
an interface to prevent passing in multiple function pointers.

sweep/interface.go

@@ -0,0 +1,12 @@
+package sweep
+
+import (
+	"github.com/btcsuite/btcd/wire"
+)
+
+// Wallet contains all wallet related functionality required by sweeper.
+type Wallet interface {
+	// PublishTransaction performs cursory validation (dust checks, etc) and
+	// broadcasts the passed transaction to the Bitcoin network.
+	PublishTransaction(tx *wire.MsgTx) error
+}