Allow basebackup name to persist across restarts.

[nicois]

About this change - What it does

Provides an additional (optional) configuration setting: durable_basebackup_path, allowing the basebackup path to be preserved across restarts of the service.

Why this way

This is backwards-compatible, allowing previous behaviour to continue as a default. When enabled, this change will allow the service to be restarted (e.g. via systemctl restart pghoard) without causing the basebackup name to be regenerated. This is helpful in a range of situations where a new basebackup should not be created, simply because the service has restarted. This durable basebackup name will be preserved indefinitely, even across server restarts.