Improve detection & handling of duplicate Node ID:

Each node on the network is supposed to have a unique cryptographic
identity. Typically, this identity is generated randomly at startup
and stored for later reuse in the (poorly named) file `wallet.db`.

If the file is copied, it is possible for two nodes to share the
same node identity. This is generally not desirable and existing
servers will detect and reject connections to other servers that
have the same key.

This commit achives three things:

1. It improves the detection code to pinpoint instances where two
   distinct servers with the same key connect with each other. In
   that case, servers will log an appropriate error and shut down
   pending intervention by the server's operator.
2. It makes it possible for server administrators to securely and
   easily generate new cryptographic identities for servers using
   the new `--newnodeid` command line arguments. When a server is
   started using this command, it will generate and save a random
   secure identity.
3. It makes it possible to configure the identity using a command
   line option, which makes it possible to derive it from data or
   parameters associated with the container or hardware where the
   instance is running by passing the `--nodeid` option, followed
   by a single argument identifying the infomation from which the
   node's identity is derived. For example, the following command
   will result in nodes with different hostnames having different
   node identities: `rippled --nodeid $HOSTNAME`

The last option is particularly useful for automated cloud-based
deployments that minimize the need for storing state and provide
unique deployment identifiers.

**Important note for server operators:**
Depending on variables outside of the the control of this code,
such as operating system version or configuration, permissions,
and more, it may be possible for other users or programs to be
able to access the command line arguments of other processes
on the system.

If you are operating in a shared environment, you should avoid
using this option, preferring instead to use the `[node_seed]`
option in the configuration file, and use permissions to limit
exposure of the node seed.

A user who gains access to the value used to derive the node's
unique identity could impersonate that node.

The commit also updates the minimum supported server protocol
version to `XRPL/2.1`, which has been supported since version
1.5.0 and eliminates support for `XPRL/2.0`.

src/test/jtx/impl/Env.cpp

@@ -83,7 +83,7 @@ Env::AppBundle::AppBundle(
         std::move(config), std::move(logs), std::move(timeKeeper_));
     app = owned.get();
     app->logs().threshold(thresh);
-    if (!app->setup())
+    if (!app->setup({}))
         Throw<std::runtime_error>("Env::AppBundle: setup failed");
     timeKeeper->set(app->getLedgerMaster().getClosedLedger()->info().closeTime);
     app->start(false /*don't start timers*/);