cache remotes via annex-speculate-present

Added remote.name.annex-speculate-present config that can be used to
make cache remotes.

Implemented it in Remote.keyPossibilities, which is used by the
get/move/copy/mirror commands, and nothing else. This way, things like
whereis will not show content that's speculatively present.

The assistant and sync --content were not using Remote.keyPossibilities,
and were changed to use it.

The efficiency hit should be small; Remote.keyPossibilities is only
used before transferring a file, which is the expensive operation.
And, it's only doing one lookup of the remoteList and a very cheap
filter over it.

Note that, git-annex still updates the location log when copying content
to a remote with annex-speculate-present set. In this case, the location
tracking will indicate that content is present in the remote. This may
not be wanted for caches, or may not be a real problem for them. TBD.

This commit was supported by the NSF-funded DataLad project.

Command/Sync.hs

@@ -616,7 +616,7 @@ seekSyncContent o rs = do
  -}
 syncFile :: Either (Maybe (Bloom Key)) (Key -> Annex ()) -> [Remote] -> AssociatedFile -> Key -> Annex Bool
 syncFile ebloom rs af k = onlyActionOn' k $ do
-	locs <- Remote.keyLocations k
+	locs <- map Remote.uuid <$> Remote.keyPossibilities k
 	let (have, lack) = partition (\r -> Remote.uuid r `elem` locs) rs
 
 	got <- anyM id =<< handleget have