feat (Bob): Buffer transfer proof to database when we are running a different swap (#1669)

swap/migrations/20210903050345_create_swaps_table.sql

@@ -22,4 +22,10 @@ CREATE TABLE if NOT EXISTS peer_addresses
 (
     peer_id     TEXT                NOT NULL,
     address     TEXT                NOT NULL
+);
+
+CREATE TABLE if NOT EXISTS buffered_transfer_proofs
+(
+    swap_id     TEXT    PRIMARY KEY NOT NULL,
+    proof       TEXT                NOT NULL
 );