postgres: add connection limit

sample-lnd.conf

@@ -1177,6 +1177,11 @@ litecoin.node=ltcd
 ; disable.
 ; db.postgres.timeout=
 
+; Postgres maximum number of connections. Set to zero for unlimited. It is
+; recommended to set a limit that is below the server connection limit.
+; Otherwise errors may occur in lnd under high-load conditions.
+; db.postgres.maxconnections=
+
 [bolt]
 
 ; If true, prevents the database from syncing its freelist to disk.