channeldb: index payments by sequence number

Add an entry to a payments index bucket which maps sequence number
to payment hash when we initiate payments. This allows for more
efficient paginated queries. We create the top level bucket in its
own migration so that we do not need to create it on the fly.

When we retry payments and provide them with a new sequence number, we
delete the index for their existing payment so that we do not have an
index that points to a non-existent payment.

If we delete a payment, we also delete its index entry. This prevents
us from looking up entries from indexes to payments that do not exist.

channeldb/migration16/log.go

@@ -0,0 +1,14 @@
+package migration16
+
+import (
+	"github.com/btcsuite/btclog"
+)
+
+// log is a logger that is initialized as disabled.  This means the package will
+// not perform any logging by default until a logger is set.
+var log = btclog.Disabled
+
+// UseLogger uses a specified Logger to output package logging info.
+func UseLogger(logger btclog.Logger) {
+	log = logger
+}