Fix API performance: one DNSSECKeeper per request

Fix API performance: one DNSSECKeeper per request

getZoneInfo() was instantiating a new DNSSECKeeper for every call. Since
DNSSECKeeper opens a new database connection, this resulted in severe
performance issues in /api/v1/servers/localhost/zones with many zones.

This is fixed by passing a DNSSECKeeper instance to getZoneInfo().
With 10,000 zones and the sqlite3 backend, this reduces the API call
execution time from ~2s to ~200ms (from ~7s to 700ms for first call).