Limit calls to maybe_add_sys_db_callbacks to once per db open

As part of performance regression testing we found out that there was a code change in
couch_server:open/2,
Options = maybe_add_sys_db_callbacks(DbName, Options0)
and as a result of that change, we are basically calling maybe_add_sys_db_callbacks for each request instead of once per db open.

We have tested a fix to ensure that we call maybe_add_sys_db_callbacks only once per db open. And executed a GET performance benchmark and found the following improvements.

• Throughput: Up to ~5% improvement.
• Up to ~22% improvement in p90 and p99 latencies