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/DropKey.hs

@@ -31,7 +31,7 @@ start key = stopUnless (inAnnex key) $ do
 	next $ perform key
 
 perform :: Key -> CommandPerform
-perform key = lockContent key $ \contentlock -> do
+perform key = lockContentExclusive key $ \contentlock -> do
 	removeAnnex contentlock
 	next $ cleanup key