add the ability to close mmapped files
fix according to code review
move the close method to the segment object
reference count to make sure we arent accessing any instances of a closed mmapped segment
additional ut for reference counting
move reference counting classes out of server manager and more tests for concurrency
put all concurrency test back  into server manager test, introduce better concurrency for reference counting
hide decrement and update close logic to call decrement
more unit tests to test for multiple closes in different threads
more data to exception msgs
Merge pull request #204 from metamx/close-handle

add the ability to unmap mmapped files