lncfg+itest: expose configurable batch-commit-interval

This will permit a greater degree of tuning or customization depending
on various hardware/environmental factors.

sample-lnd.conf

@@ -930,6 +930,10 @@ litecoin.node=ltcd
 ; also has experimental support for etcd, a replicated backend.
 ; db.backend=bolt
 
+; The maximum interval the graph database will wait between attempting to flush
+; a batch of modifications to disk. Defaults to 500 milliseconds.
+; db.batch-commit-interval=500ms
+
 [etcd]
 ; Etcd database host.
 ; db.etcd.host=localhost:2379