configurable sleep time for Litestream.verify!

[spinosa]

When sync-interval is configured beyond the default, Litestream.verify! may appear to fail where it actually didn't wait long enough for replication.

This adds a parameter replication_sleep: to Litestream.verify! with a default of 10 for backwards compatibility.

While reading litestream.yml configuration to parse sync-interval and using a multiple of it might be nice, the added complexity didn't seem justified.