Remove default ctors from SecretKey and PublicKey: (#4607)

* It is now an invariant that all constructed Public Keys are valid,
  non-empty and contain 33 bytes of data.
* Additionally, the memory footprint of the PublicKey class is reduced.
  The size_ data member is declared as static.
* Distinguish and identify the PublisherList retrieved from the local
  config file, versus the ones obtained from other validators.
* Fixes #2942

src/test/jtx/Account.h

@@ -46,7 +46,7 @@ public:
     /** The master account. */
     static Account const master;
 
-    Account() = default;
+    Account() = delete;
     Account(Account&&) = default;
     Account(Account const&) = default;
     Account&

src/test/jtx/Env_test.cpp

@@ -50,7 +50,7 @@ public:
     {
         using namespace jtx;
         {
-            Account a;
+            Account a("chenna");
             Account b(a);
             a = b;
             a = std::move(b);