Implemented StringAppendOperator and unit tests.

Summary:
Implemented the StringAppendOperator class (subclass of MergeOperator).
Found in utilities/merge_operators/string_append/stringappend.{h,cc}

It is a rocksdb Merge Operator that supports string/list concatenation
 with a configurable delimiter.

The tests are found in .../stringappend_test.cc. It implements a
 map : key -> (list of strings), with core operations Append(list_key,val)
 and Get(list_key).

Test Plan:
1. Navigate to your rocksdb repository
2. Execute: make stringappend_test  (to compile)
3. Execute: ./stringappend_test (to run the tests)
4. Execute: make all check (to test the ENTIRE rocksdb codebase / regression)

Reviewers: haobo, dhruba, zshao

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10737

utilities/merge_operators/string_append/stringappend.cc

@@ -0,0 +1,57 @@
+/**
+ * A MergeOperator for rocksdb/leveldb that implements string append.
+ * @author Deon Nicholas (dnicholas@fb.com)
+ * Copyright 2013 Facebook
+ */
+
+#include "stringappend.h"
+
+#include <memory>
+#include <assert.h>
+#include "leveldb/slice.h"
+#include "leveldb/merge_operator.h"
+#include "utilities/merge_operators.h"
+#include <iostream>
+
+namespace leveldb {
+
+
+// Constructor: also specify the  delimiter character.
+StringAppendOperator::StringAppendOperator(char delim_char)
+    : delim_(delim_char) {
+}
+
+// Implementation for the merge operation (concatenates two strings)
+void StringAppendOperator::Merge(const Slice& key,
+                                 const Slice* existing_value,
+                                 const Slice& value,
+                                 std::string* new_value,
+                                 Logger* logger) const {
+
+  // Clear the *new_value for writing.
+  assert(new_value);
+  new_value->clear();
+
+  if (!existing_value) {
+    // No existing_value. Set *new_value = value
+    new_value->assign(value.data(),value.size());
+  } else {
+    // Generic append (existing_value != null).
+    // Reserve *new_value to correct size, and apply concatenation.
+    new_value->reserve(existing_value->size() + 1 + value.size());
+    new_value->assign(existing_value->data(),existing_value->size());
+    new_value->append(1,delim_);
+    new_value->append(value.data(), value.size());
+  }
+
+  return;
+}
+
+const char* StringAppendOperator::Name() const  {
+  return "StringAppendOperator";
+}
+
+} // namespace leveldb
+
+
+