finalize RepoSize database

Including locking on creation, handling of permissions errors, and
setting repo sizes.

I'm confident that locking is not needed while using this database.
Since writes happen in a single transaction. When there are two writers
that are recording sizes based on different git-annex branch commits,
one will overwrite what the other one recorded. Which is fine, it's only
necessary that the database stays consistent with the content of a
git-annex branch commit.

Annex/Locations.hs

@@ -76,6 +76,7 @@ module Annex.Locations (
 	gitAnnexImportFeedDbDir,
 	gitAnnexImportFeedDbLock,
 	gitAnnexRepoSizeDbDir,
+	gitAnnexRepoSizeDbLock,
 	gitAnnexScheduleState,
 	gitAnnexTransferDir,
 	gitAnnexCredsDir,
@@ -521,6 +522,10 @@ gitAnnexRepoSizeDbDir :: Git.Repo -> GitConfig -> RawFilePath
 gitAnnexRepoSizeDbDir r c =
 	fromMaybe (gitAnnexDir r) (annexDbDir c) P.</> "reposize"
 
+{- Lock file for the reposize database. -}
+gitAnnexRepoSizeDbLock :: Git.Repo -> GitConfig -> RawFilePath
+gitAnnexRepoSizeDbLock r c = gitAnnexRepoSizeDbDir r c <> ".lck"
+
 {- .git/annex/schedulestate is used to store information about when
  - scheduled jobs were last run. -}
 gitAnnexScheduleState :: Git.Repo -> RawFilePath