Performance degraded when concurrent requests are using the same user

[jhanders34]

Describe the bug
When concurrent requests to Liberty use the same user to authenticate, operations against the Subject generated can bottleneck on the same Subject when using the builtin authentication cache.

Same stack from a thread dump showing where applications are bottlenecked is:

 at javax.security.auth.Subject$ClassSet.<init>(Ljavax/security/auth/Subject;ILjava/lang/Class;)V (Subject.java:1667(Compiled Code))
 at javax.security.auth.Subject.getPublicCredentials(Ljava/lang/Class;)Ljava/util/Set; (Subject.java:910(Compiled Code))
 at com.ibm.websphere.security.auth.WSSubject.getWSCredential(Ljavax/security/auth/Subject;)Lcom/ibm/websphere/security/cred/WSCredential; (WSSubject.java:625(Compiled Code))

Steps to Reproduce
Run concurrent requests against a Liberty server with authentication enabled and the in memory authentication cache enabled and use the same user for each of the requests.

Expected behavior
Concurrent requests with the same user, should be able to run concurrently and not get bottlenecked on each other.

Diagnostic information:

• OpenLiberty Version: 8.5.5 - 26.0.0.2
• Java Version: All

Additional context
This problem can be worked around by disabling the authentication cache, but that can lead to other performance problems.