* dfs: use ReftableConfig() constructor in DfsReftableDatabase
* dfs: use RefDatabase API for exactRef in ReftableBatchRefUpdate
* reftable: add OutputStream argument to ReftableWriter constructor
* reftable: read file footer in ReftableReader#allRefs
* reftable: add ReftableReflogReader
* UploadPack: Use reachability checker to validate non-advertised wants
* BitmappedReachabilityChecker: Reachability check using bitmaps
* BitmapCalculator: Get the reachability bitmap of a commit
* ReachabilityChecker: Default implementation with a RevWalk