Fixed getCachedExecutor race condition in MethodReference

This commit includes a full backport of the changes in SPR-10657, instead of just the initial commit which the original backport request referred to.

Issue: SPR-10884
Issue: SPR-10657
Properly handle getMethodDescriptors() null value on IBM JVM

Also internally renaming SimpleNonIndexedPropertyDescriptor to SimplePropertyDescriptor and preferring direct field access wherever possible.

Issue: SPR-10862
Fixed method count in AbstractMethodMockingControl's IllegalStateException message

Issue: SPR-10885