Add snapshot interval

This commit adds the ability to periodically perform snapshots on
an interval that is separate from retention. For example, this lets
you retain backups for 24 hours but you can snapshot your database
every six hours to improve recovery time.

cmd/litestream/main.go

@@ -243,6 +243,7 @@ type ReplicaConfig struct {
 	Retention              time.Duration `yaml:"retention"`
 	RetentionCheckInterval time.Duration `yaml:"retention-check-interval"`
 	SyncInterval           time.Duration `yaml:"sync-interval"` // s3 only
+	SnapshotInterval       time.Duration `yaml:"snapshot-interval"`
 	ValidationInterval     time.Duration `yaml:"validation-interval"`
 
 	// S3 settings
@@ -304,6 +305,9 @@ func newFileReplicaFromConfig(c *ReplicaConfig, db *litestream.DB) (_ *litestrea
 	if v := c.RetentionCheckInterval; v > 0 {
 		r.RetentionCheckInterval = v
 	}
+	if v := c.SnapshotInterval; v > 0 {
+		r.SnapshotInterval = v
+	}
 	if v := c.ValidationInterval; v > 0 {
 		r.ValidationInterval = v
 	}
@@ -372,6 +376,9 @@ func newS3ReplicaFromConfig(c *ReplicaConfig, db *litestream.DB) (_ *s3.Replica,
 	if v := c.SyncInterval; v > 0 {
 		r.SyncInterval = v
 	}
+	if v := c.SnapshotInterval; v > 0 {
+		r.SnapshotInterval = v
+	}
 	if v := c.ValidationInterval; v > 0 {
 		r.ValidationInterval = v
 	}