Time-to-live configuration for automatic document expiration.
Uses a MongoDB TTL index on the expiresAt field. Each document's
expiresAt is set to now + defaultTtl on every put (and optionally
on every get when refreshOnRead is true). MongoDB's background thread
automatically removes documents once expiresAt has passed.
Requires MongoDBStore.start to be called to create the TTL index.
Default TTL in seconds
Whether to reset TTL timer on get (extends life of frequently accessed items)