multi: Add decayedlog db migration code

This commit adds the migration code for the decayedlog db which
is optional and will default to true.

sample-lnd.conf

@@ -1476,6 +1476,11 @@
 ; channels prior to lnd@v0.15.0.
 ; db.prune-revocation=false
 
+; Specify whether the optional migration for garbage collecting the decayed
+; sphinx logs should be applied. By default, the decayed log will be garbage
+; collected.
+; db.no-gc-decayed-log=false
+
 ; If set to true, then the to-local and to-remote output amount data of revoked
 ; commitment transactions will not be stored in the revocation log. Note that
 ; this flag can only be set if --wtclient.active is not set. It is not