sweep+lnrpc: add new interface FeePreference

This commit adds a new interface `FeePreference` which makes it easier
to write unit tests and allows more customized implementation in
following commits.

sweep/sweeper.go

@@ -68,7 +68,7 @@ type Params struct {
 	// Fee is the fee preference of the client who requested the input to be
 	// swept. If a confirmation target is specified, then we'll map it into
 	// a fee rate whenever we attempt to cluster inputs for a sweep.
-	Fee FeeEstimateInfo
+	Fee FeePreference
 
 	// Force indicates whether the input should be swept regardless of
 	// whether it is economical to do so.
@@ -84,7 +84,7 @@ type ParamsUpdate struct {
 	// Fee is the fee preference of the client who requested the input to be
 	// swept. If a confirmation target is specified, then we'll map it into
 	// a fee rate whenever we attempt to cluster inputs for a sweep.
-	Fee FeeEstimateInfo
+	Fee FeePreference
 
 	// Force indicates whether the input should be swept regardless of
 	// whether it is economical to do so.