SERVER-13638: remove NamespaceDetails from record store entirely
SERVER-13639: make Extent only have read methods
SERVER-13639: make Extent a Struct
SERVER-13638: remove system flags from NamespaceDetails
SERVER-13639: clean up extent.* includes
SERVER-13639: add ExtentManager interface
SERVER-13639: small changes to extent code
Merge remote-tracking branch 'upstream/master'