SERVER-6405: make ExtentManager not use global directoryperdb
SERVER-6405: DiskLoc::assertOk const
SERVER-6405: add ExtentManager::getExtent
SERVER-6405: re-arranging DataFile, Database and ExtentManager to encapsulate Extent logic better
SERVER-6405: fix some spacing
SERVER-6405: move db/record* to db/storage/record.*
SERVER-6405: add ExtentManager::get(Next|Prev)Extent
SERVER-6405: put collection.h in a new directory mongo/db/structure