Refactor AccountID (RIPD-953):

All AccountID functionality is removed from RippleAddress and
replaced with free functions. The AccountID to string conversion
cache is factored out as an explicit type with an instance in
the Application object. New base58 conversion functions are used,
with no dependence on OpenSSL.

All types and free functions related to AccountID are consolidated
into one header file. Routines to operate on "tokens" are also
introduced and consolidated into a single header file.

A token one of the cryptographic primitives used in Ripple:

    Secret Seed
    Server Public Key
    Server Secret Key
    Account ID
    Account Public Key
    Account Private Key

    and these deprecated primitives:

    Account Family Seed
    Account Family Generator

src/ripple/app/main/Application.cpp

@@ -58,6 +58,7 @@
 #include <ripple/overlay/make_Overlay.h>
 #include <ripple/protocol/Indexes.h>
 #include <ripple/protocol/STParsedJSON.h>
+#include <ripple/protocol/types.h>
 #include <ripple/rpc/Manager.h>
 #include <ripple/server/make_ServerHandler.h>
 #include <ripple/shamap/Family.h>
@@ -259,6 +260,7 @@ public:
     std::unique_ptr <SHAMapStore> m_shaMapStore;
     std::unique_ptr <NodeStore::Database> m_nodeStore;
     PendingSaves pendingSaves_;
+    AccountIDCache accountIDCache_;
 
     // These are not Stoppable-derived
     NodeCache m_tempNodeCache;
@@ -334,6 +336,8 @@ public:
 
         , m_nodeStore (m_shaMapStore->makeDatabase ("NodeStore.main", 4))
 
+        , accountIDCache_(128000)
+
         , m_tempNodeCache ("NodeCache", 16384, 90, get_seconds_clock (),
             m_logs.journal("TaggedCache"))
 
@@ -598,6 +602,12 @@ public:
         return pendingSaves_;
     }
 
+    AccountIDCache const&
+    accountIDCache() const override
+    {
+        return accountIDCache_;
+    }
+
     Overlay& overlay ()
     {
         return *m_overlay;
@@ -904,7 +914,6 @@ public:
 
         m_overlay->saveValidatorKeyManifests (getWalletDB ());
 
-        RippleAddress::clearCache ();
         stopped ();
     }
 
@@ -1038,11 +1047,12 @@ void ApplicationImp::startNewLedger ()
 
     // Print enough information to be able to claim root account.
     m_journal.info << "Root master seed: " << rootSeedMaster.humanSeed ();
-    m_journal.info << "Root account: " << rootAddress.humanAccountID ();
+    m_journal.info << "Root account: " << toBase58(calcAccountID(rootAddress));
 
     {
         Ledger::pointer firstLedger = std::make_shared<Ledger> (rootAddress, SYSTEM_CURRENCY_START);
-        assert (firstLedger->exists(keylet::account(rootAddress.getAccountID())));
+        assert (firstLedger->exists(keylet::account(
+            calcAccountID(rootAddress))));
         // TODO(david): Add any default amendments
         // TODO(david): Set default fee/reserve
         firstLedger->getHash(); // updates the hash
@@ -1054,7 +1064,8 @@ void ApplicationImp::startNewLedger ()
         secondLedger->setClosed ();
         secondLedger->setAccepted ();
         m_ledgerMaster->pushLedger (secondLedger, std::make_shared<Ledger> (true, std::ref (*secondLedger)));
-        assert (secondLedger->exists(keylet::account(rootAddress.getAccountID())));
+        assert (secondLedger->exists(keylet::account(
+            calcAccountID(rootAddress))));
         m_networkOPs->setLastCloseTime (secondLedger->getCloseTimeNC ());
     }
 }