Add a defragmentation mechanism

Add a defragmentation mechanism

From the ML: (paliwalashish)
>Will the offHeapMemoryBuffer get fragmented over time? Say after a
couple thousand get/remove operations, will the off-heap have start
having holes in the Buffer?
(Me
>It will, definitely. I had two solutions ready in my mind (that rely on having more than one buffer active):
Simplest, and fastest but with some drawbacks: when buffer.isTooDefragmented() then simply buffer.clear() - you loose everything, but - hey, it's a cache, not a db
Less simple, slower, less drawbacks: when buffer.isTooDefragmented() mark the buffer as readOnly and then foreach (ptr in buffer) copy ptr.content in emptyBuffer and update ptr accordingly
where isTooFragmented==number_of_empty_pointers over total_pointers > desirable quota

The first one could be accomplished during a put() operation (buffer.clear is a logical operation that takes no time) while the second should be taken care of by the background thread. Those quick&dirty solutions could of course be replaced with real defragmentation algorithms - may taken from various malloc() implementations, that are the original inspiration http://en.wikipedia.org/wiki/Malloc#Implementations

See also https://github.com/raffaeleguidi/DirectMemory/issues/43