MBeanExporter: memory leak related to ManagedClasses registry

[malayerba]

ManagedClasses that are registered in exportedManagedClasses are never unregistered. This is causing an accumulation of memory.

[mehdi-khosravi]

+1, this ConcurrentHashmap grows gradually and causes high heap usage.
Created this PR #55

[luke-sterkowicz]

+1