mod - idempotent migration file.

the handwritten migration file itself is silly, persistent should take care of it.
but while its here it is a lot less problematic if you use "if not exists" statements, as it allows
db-sync to startup without crashing.
we could also use the migration version numbers as found in the other migration files, but I opted
not to do that because I was worried about breaking things for other migrations.

schema/migration-4-0001-20200605.sql

@@ -17,11 +17,10 @@
 CREATE FUNCTION migrate() RETURNS void AS $$
 
 BEGIN
-  EXECUTE 'create table tx_body (id serial8 primary key unique, 
-                                 hash hash32type not null, 
+  EXECUTE 'create table if not exists tx_body (id serial8 primary key unique, 
+                                 hash hash32type not null unique, 
                                  body bytea not null)';
-  EXECUTE 'CREATE INDEX idx_tx_body_hash ON tx_body(hash);';
-  EXECUTE 'ALTER TABLE "tx_body" ADD CONSTRAINT "unique_tx_body" UNIQUE("hash")' ;
+  EXECUTE 'CREATE INDEX if not exists idx_tx_body_hash ON tx_body(hash);';
 END;
 
 $$ LANGUAGE plpgsql;