[tail sampling] Review behaviour when Elasticsearch license checks fail

[axw]

Tail-based sampling currently requires a platinum license. We should ensure that when Elasticsearch is inaccessible, APM Server does not exit but instead retries requests later, particularly in case of intermittent network failures.

If the Elasticsearch request succeeds but the license is incompatible, then we set some kind of status indicator in Fleet, or reject the received Fleet policy outright.

[axw]

Running apm-server standalone with tail-based sampling enabled, the server exits immediately if Elasticsearch is unavailable as the server tries and fails to create a data stream for sampling decisions. Easily reproduced with:

./apm-server -e --strict.perms=false -E 'apm-server.sampling.tail.policies=[{sample_rate: 0.1}]'

[axw]

This will be fixed by #5952, which removes the data stream creation from standalone apm-server.