When a message is shared, share its transitive dependencies

Like other recursive operations on the dependency graph, this is
not done in a single transaction to prevent an attacker from creating
arbitrary large transactions.

So at startup, the `ValidationManager` finds and resumes any
unfinished operations, by looking for shared messages with unshared
dependencies.

briar-tests/src/org/briarproject/blogs/BlogManagerImplTest.java

@@ -199,7 +199,6 @@ public class BlogManagerImplTest extends BriarTestCase {
 		);
 
 		context.checking(new Expectations() {{
-			oneOf(clientHelper).setMessageShared(txn, messageId, true);
 			oneOf(identityManager)
 					.getAuthorStatus(txn, blog1.getAuthor().getId());
 			will(returnValue(VERIFIED));