add lockContentShared

Also, rename lockContent to lockContentExclusive

inAnnexSafe should perhaps be eliminated, and instead use
`lockContentShared inAnnex`. However, I'm waiting on that, as there are
only 2 call sites for inAnnexSafe and it's fiddly.

Command/Drop.hs

@@ -88,12 +88,12 @@ startRemote afile numcopies key remote = do
 	showStart' ("drop " ++ Remote.name remote) key afile
 	next $ performRemote key afile numcopies remote
 
--- Note that lockContent is called before checking if the key is present
--- on enough remotes to allow removal. This avoids a scenario where two
+-- Note that lockContentExclusive is called before checking if the key is
+-- present on enough remotes to allow removal. This avoids a scenario where two
 -- or more remotes are trying to remove a key at the same time, and each
 -- see the key is present on the other.
 performLocal :: Key -> AssociatedFile -> NumCopies -> Maybe Remote -> CommandPerform
-performLocal key afile numcopies knownpresentremote = lockContent key $ \contentlock -> do
+performLocal key afile numcopies knownpresentremote = lockContentExclusive key $ \contentlock -> do
 	(remotes, trusteduuids) <- Remote.keyPossibilitiesTrusted key
 	let trusteduuids' = case knownpresentremote of
 		Nothing -> trusteduuids