Add buffer pools to System Performance tool

[AFaust]

FEATURE / ENHANCEMENT

Any custom / 3rd party feature added to Alfresco that is based on Java code may at any point make use of Java NIO features for direct memory allocation or mapped byte buffers, which will not be tracked via the current heap monitoring graph. I personally have developed a custom caching solution that supports off-heap storage of cache entries. Memory used by off-heap storage is currently not shown anywhere in the System Performance tool, and even standard monitoring tools may not show it by default (e.g. jvisualvm requires an additional plugin to show buffer pools). Since the information is available via a local MBean we should be able to easily access and display the metrics.