Make --wait-for-supermajority require --expected-shred-version (#192)

In cluster restart scenarios, an important step is scanning the
Blockstore for blocks that occur after the chosen restart slot with an
incorrect shred version. This check ensures that any blocks that
occurred pre-cluster restart and after the chosen restart slot get
deleted. If a node skips this step, the node can encounter problems when
that block is created again, after the cluster has restarted.

This check only occurs if --wait-for-supermajority AND
--expected-shred-version are set; however, --expected-... is currently
optional when using --wait-...

Our restart instructions typically mention that one should specify
--expected-... as well, but we should just enforce it at the CLI level
to prevent mistakes / wasted time debuggging.

validator/src/cli.rs

@@ -745,6 +745,7 @@ pub fn app<'a>(version: &'a str, default_args: &'a DefaultArgs) -> App<'a, 'a> {
             Arg::with_name("wait_for_supermajority")
                 .long("wait-for-supermajority")
                 .requires("expected_bank_hash")
+                .requires("expected_shred_version")
                 .value_name("SLOT")
                 .validator(is_slot)
                 .help(