health check error when present internal errors

## Summary
It will be fine to define settings like
> 1. internal_error_max_rate - example 0.001, which means if 0.1% of the requests failed with 5xx error code, then trigger healthcheck_fail.
> 2. healthcheck_retry_timeout - example 600 seconds, how long to wait after the last internal error before returning "200 OK" for healthcheck.

This will allow to remove the failed CouchDB node for the request distribution on load balancers like AWS ALB.

Example when CouchDB health check returns "200 OK" when the node cannot pull a shard on the local filesystem
https://github.com/apache/couchdb/issues/4790

Start by reviewing CouchDB's health-check handling and the internal-error path described in the issue, then compare the behavior with linked issue #4790. Done means defining the requested error-rate and retry-timeout settings so health checks fail during qualifying 5xx errors and return 200 OK after the timeout; no source file or test is named.

Written by the indexing model from the issue text.