This commit adds the configuration `auto_flush_timeout` to
the gitaudit-configuration. If `auto_flush_timeout` seconds
have passed without adding a new audit log entry and there
is a non-empty changeset, i.e., the changeset was last
touched more than `auto_flush_timeout` seconds ago, the
changeset will be persisted. This results in a commit.
For example, if `auto_flush_timeout` is 60, but every 50
seconds a log-entry for a new record is added to the
gitaudit-instance, the automated persisting of the
changeset will never be triggered (the changeset might
still be persisted, depending on the content of the
changes).
If, on the other hand, a single change was added and
no other change is added in the next `auto_flush_timeout`
seconds, the single change will be persisted,
resulting in a commit with a single change.
