make sure all sqlite selects have indexes

Bearing in mind that these indexes are really uniqueness constraints
that just happen to also make sqlite generate indexes.

In Database.ContentIndentifier, the ContentIndentifiersKeyRemoteCidIndex
is fine as a uniqueness constraint because it contains all rows from the
table. The ContentIndentifiersCidRemoteIndex is also ok because there
can only be one key for a given (cid, uuid) combination.

In Database.Export, the new ExportTreeFileKeyIndex is the same pair as
the old ExportTreeKeyFileIndex (previously ExportTreeIndex). And
in Database.Keys.SQL, the new InodeCacheKeyIndex is the same pair as the
old KeyInodeCacheIndex.

Database/Keys/SQL.hs

@@ -52,6 +52,7 @@ Content
   key Key
   inodecache InodeCache
   KeyInodeCacheIndex key inodecache
+  InodeCacheKeyIndex inodecache key
 |]
 
 containedTable :: TableName