APIv2: remove tx_history and ledger_header (#4759)

Remove `tx_history` and `ledger_header` methods from API version 2.

Update `RPC::Handler` to allow for methods (or method implementations)
to be API version specific. This partially resolves #4727. We can now
store multiple handlers with the same name, as long as they belong to
different (non-overlapping) API versions. This necessarily impacts the
handler lookup algorithm and its complexity; however, there is no
performance loss on x86_64 architecture, and only minimal performance
loss on arm64 (around 10ns). This design change gives us extra
flexibility evolving the API in the future, including other parts of
#4727.

In API version 2, `tx_history` and `ledger_header` are no longer
recognised; if they are called, `rippled` will return error
`unknownCmd`

Resolve #3638

Resolve #3539

src/test/rpc/TransactionHistory_test.cpp

@@ -54,6 +54,21 @@ class TransactionHistory_test : public beast::unit_test::suite
         }
     }
 
+    void
+    testCommandRetired()
+    {
+        testcase("Command retired from API v2");
+        using namespace test::jtx;
+        Env env{*this, envconfig(no_admin)};
+
+        Json::Value params{Json::objectValue};
+        params[jss::api_version] = 2;
+        auto const result =
+            env.client().invoke("tx_history", params)[jss::result];
+        BEAST_EXPECT(result[jss::error] == "unknownCmd");
+        BEAST_EXPECT(result[jss::status] == "error");
+    }
+
     void
     testRequest()
     {
@@ -148,6 +163,7 @@ public:
     {
         testBadInput();
         testRequest();
+        testCommandRetired();
     }
 };