Update server reload to not reset aggregation/sampling

Reloading the apm-server config under fleet currently utilizes `SO_REUSEPORT` to start a second server with the updated config that listens on the same port, and then shuts down the old server.

As mentioned in [this comment](
https://github.com/elastic/apm-server/pull/5897#pullrequestreview-724974474), this approach has the undesirable effect of resetting aggregation and sampling on each config reload.

Figure out a way to persist sampling and aggregation through reloads.