Refactor and improve the SHAMap code:

This commit combines a number of cleanups, targeting both the
code structure and the code logic. Large changes include:

 - Using more strongly-typed classes for SHAMap nodes, instead of relying
   on runtime-time detection of class types. This change saves 16 bytes
   of memory per node.
 - Improving the interface of SHAMap::addGiveItem and SHAMap::addItem to
   avoid the need for passing two bool arguments.
 - Documenting the "copy-on-write" semantics that SHAMap uses to
   efficiently track changes in individual nodes.
 - Removing unused code and simplifying several APIs.
 - Improving function naming.

src/ripple/shamap/impl/SHAMapLeafNode.cpp

@@ -0,0 +1,94 @@
+//------------------------------------------------------------------------------
+/*
+    This file is part of rippled: https://github.com/ripple/rippled
+    Copyright (c) 2012, 2013 Ripple Labs Inc.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose  with  or without fee is hereby granted, provided that the above
+    copyright notice and this permission notice appear in all copies.
+
+    THE  SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+    WITH  REGARD  TO  THIS  SOFTWARE  INCLUDING  ALL  IMPLIED  WARRANTIES  OF
+    MERCHANTABILITY  AND  FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+    ANY  SPECIAL ,  DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+    WHATSOEVER  RESULTING  FROM  LOSS  OF USE, DATA OR PROFITS, WHETHER IN AN
+    ACTION  OF  CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+    OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+*/
+//==============================================================================
+
+#include <ripple/basics/contract.h>
+#include <ripple/beast/core/LexicalCast.h>
+#include <ripple/shamap/SHAMapLeafNode.h>
+
+namespace ripple {
+
+SHAMapLeafNode::SHAMapLeafNode(
+    std::shared_ptr<SHAMapItem const> item,
+    std::uint32_t cowid)
+    : SHAMapTreeNode(cowid), item_(std::move(item))
+{
+    assert(item_->peekData().size() >= 12);
+}
+
+SHAMapLeafNode::SHAMapLeafNode(
+    std::shared_ptr<SHAMapItem const> item,
+    std::uint32_t cowid,
+    SHAMapHash const& hash)
+    : SHAMapTreeNode(cowid, hash), item_(std::move(item))
+{
+    assert(item_->peekData().size() >= 12);
+}
+
+std::shared_ptr<SHAMapItem const> const&
+SHAMapLeafNode::peekItem() const
+{
+    return item_;
+}
+
+bool
+SHAMapLeafNode::setItem(std::shared_ptr<SHAMapItem const> i)
+{
+    assert(cowid_ != 0);
+    item_ = std::move(i);
+
+    auto const oldHash = hash_;
+
+    updateHash();
+
+    return (oldHash != hash_);
+}
+
+std::string
+SHAMapLeafNode::getString(const SHAMapNodeID& id) const
+{
+    std::string ret = SHAMapTreeNode::getString(id);
+
+    auto const type = getType();
+
+    if (type == SHAMapNodeType::tnTRANSACTION_NM)
+        ret += ",txn\n";
+    else if (type == SHAMapNodeType::tnTRANSACTION_MD)
+        ret += ",txn+md\n";
+    else if (type == SHAMapNodeType::tnACCOUNT_STATE)
+        ret += ",as\n";
+    else
+        ret += ",leaf\n";
+
+    ret += "  Tag=";
+    ret += to_string(item_->key());
+    ret += "\n  Hash=";
+    ret += to_string(hash_);
+    ret += "/";
+    ret += std::to_string(item_->size());
+    return ret;
+}
+
+void
+SHAMapLeafNode::invariants(bool) const
+{
+    assert(hash_.isNonZero());
+    assert(item_ != nullptr);
+}
+
+}  // namespace ripple