[UNOMI-924] Make healthcheck concurrent

[jayblanc]

This pull request refactors the HealthCheckService to improve thread safety, caching, and concurrency, as well as to simplify the health check execution logic. The main changes include introducing a cache for health check results, switching to a cached thread pool, and removing unnecessary stream usage.

Thread safety and caching improvements:

• Added a cache (healthCache) for health check results, a timestamp (cacheTimestamp), and synchronization logic to prevent concurrent refreshes. This ensures results are not recomputed too frequently and improves performance under load. [1] [2]
• Introduced a cacheLock object and busy flag to coordinate cache refreshes and avoid race conditions. [1] [2]

Concurrency and execution changes:

• Changed the executor from a single-threaded executor to a cached thread pool to better handle concurrent health check provider executions.
• Removed the use of Future and timeouts when executing health check providers; now calls provider.execute() directly and handles exceptions by marking the provider as timed out.

Code simplification and cleanup:

• Removed unnecessary use of streams and collectors when filtering providers, simplifying the code with a direct toList() call. [1] [2]
• Improved logging and early returns when the health check service is disabled.

• I hereby declare this contribution to be licensed under the Apache License Version 2.0, January 2004