Review ConcurrentHashMap usage in helidon-common-configurable

[spericas]

Problem Description

Thread pinning can occur if the mapping function passed to any of the compute methods in ConcurrentHashMap results in any I/O. The internal implementation of ConcurrentHashMap will hold a lock while the VT is parked, causing the pinning.

Note that I/O in the mapping function can be indirect: for example, using config or a service loader. Please verify that the following calls will never need I/O to complete:

ObserverManager:76

If required, re-implement the map using a traditional reentrant lock and a hash map.

[tjquinno]

A quick trace of the downstream code shows that the compute-if-absent function does indeed use service loading, so this will probably need to be rewritten using a lock.