Replace base_uint(string) with from_hex_text<> (RIPD-897)

Removes the base_uint constructor that took a string.  Replaces
that functionality with two free functions named from_hex_text<>.
Use of from_hex_text<> looks like this:

auto v = from_hex_text<uint256>("AAA555");
static_assert (std::is_same<decltype(v), uint256>::value, "Huh!");

from_hex_text<> only operates on base_uint types.  At the moment the
list of those types include:

 o uint128,
 o uint160,
 o uint256,
 o Directory,
 o Account,
 o Currency, and
 o NodeID.

Using from_hex_text<> with any other types will not compile due to
an enable_if.

src/ripple/nodestore/backend/RocksDBQuickFactory.cpp

@@ -136,7 +136,7 @@ public:
 
         // overrride OptimizeLevelStyleCompaction
         options.min_write_buffer_number_to_merge = 1;
-        
+
         rocksdb::BlockBasedTableOptions table_options;
         // Use hash index
         table_options.index_type =
@@ -145,7 +145,7 @@ public:
             rocksdb::NewBloomFilterPolicy(10));
         options.table_factory.reset(
             NewBlockBasedTableFactory(table_options));
-        
+
         // Higher values make reads slower
         // table_options.block_size = 4096;
 
@@ -273,7 +273,7 @@ public:
     storeBatch (Batch const& batch)
     {
         rocksdb::WriteBatch wb;
- 
+
         EncodedBlob encoded;
 
         for (auto const& e : batch)
@@ -291,7 +291,7 @@ public:
 
         // Crucial to ensure good write speed and non-blocking writes to memtable
         options.disableWAL = true;
-        
+
         auto ret = m_db->Write (options, &wb);
 
         if (!ret.ok ())
@@ -321,7 +321,8 @@ public:
                 {
                     // Uh oh, corrupted data!
                     if (m_journal.fatal) m_journal.fatal <<
-                        "Corrupt NodeObject #" << uint256 (it->key ().data ());
+                        "Corrupt NodeObject #" <<
+                        from_hex_text<uint256>(it->key ().data ());
                 }
             }
             else