Closes Tencent#1643

This change comes up with compile time pre-processor directives to
tune the behavior of rapidjson wrt memory consumption. The idea is to
allow each module using this library to choose the right defaults based
on how it consumes memory and what performance it expects.

1. RAPIDJSON_DEFAULT_ALLOCATOR: If defined allows you to choose
	CrtAllocator over MemoryPoolAllocator. If it is not defined, chooses MemoryPoolAllocator by default.
2. RAPIDJSON_DEFAULT_STACK_ALLOCATOR: If defined allows you to choose
	MemoryPoolAllocator over CrtAllocator. If it is not defined, chooses CrtAllocator by default.
3. RAPIDJSON_VALUE_DEFAULT_OBJECT_CAPACITY and RAPIDJSON_VALUE_DEFAULT_ARRAY_CAPACITY: If defined and set to a
	value, uses that value for default number of objects/array elements to be pre-allocated. If not defined,
	uses value of 16: the current default.

Verified that all tests pass.

include/rapidjson/document.h

@@ -56,6 +56,48 @@ class GenericValue;
 template <typename Encoding, typename Allocator, typename StackAllocator>
 class GenericDocument;
 
+/*! \def RAPIDJSON_DEFAULT_ALLOCATOR
+    \ingroup RAPIDJSON_CONFIG
+    \brief Allows to choose default allocator.
+
+    User can define this to use CrtAllocator or MemoryPoolAllocator.
+*/
+#ifndef RAPIDJSON_DEFAULT_ALLOCATOR
+#define RAPIDJSON_DEFAULT_ALLOCATOR MemoryPoolAllocator<CrtAllocator>
+#endif
+
+/*! \def RAPIDJSON_DEFAULT_STACK_ALLOCATOR
+    \ingroup RAPIDJSON_CONFIG
+    \brief Allows to choose default stack allocator for Document.
+
+    User can define this to use CrtAllocator or MemoryPoolAllocator.
+*/
+#ifndef RAPIDJSON_DEFAULT_STACK_ALLOCATOR
+#define RAPIDJSON_DEFAULT_STACK_ALLOCATOR CrtAllocator
+#endif
+
+/*! \def RAPIDJSON_VALUE_DEFAULT_OBJECT_CAPACITY
+    \ingroup RAPIDJSON_CONFIG
+    \brief User defined kDefaultObjectCapacity value.
+
+    User can define this as any natural number.
+*/
+#ifndef RAPIDJSON_VALUE_DEFAULT_OBJECT_CAPACITY
+// number of objects that rapidjson::Value allocates memory for by default
+#define RAPIDJSON_VALUE_DEFAULT_OBJECT_CAPACITY 16
+#endif
+
+/*! \def RAPIDJSON_VALUE_DEFAULT_ARRAY_CAPACITY
+    \ingroup RAPIDJSON_CONFIG
+    \brief User defined kDefaultArrayCapacity value.
+
+    User can define this as any natural number.
+*/
+#ifndef RAPIDJSON_VALUE_DEFAULT_ARRAY_CAPACITY
+// number of array elements that rapidjson::Value allocates memory for by default
+#define RAPIDJSON_VALUE_DEFAULT_ARRAY_CAPACITY 16
+#endif
+
 //! Name-value pair in a JSON object value.
 /*!
     This class was internal to GenericValue. It used to be a inner struct.
@@ -604,7 +646,7 @@ template <bool, typename> class GenericObject;
     \tparam Encoding    Encoding of the value. (Even non-string values need to have the same encoding in a document)
     \tparam Allocator   Allocator type for allocating memory of object, array and string.
 */
-template <typename Encoding, typename Allocator = MemoryPoolAllocator<> > 
+template <typename Encoding, typename Allocator = RAPIDJSON_DEFAULT_ALLOCATOR >
 class GenericValue {
 public:
     //! Name-value pair in an object.
@@ -1969,8 +2011,8 @@ class GenericValue {
         kTypeMask = 0x07
     };
 
-    static const SizeType kDefaultArrayCapacity = 16;
-    static const SizeType kDefaultObjectCapacity = 16;
+    static const SizeType kDefaultArrayCapacity = RAPIDJSON_VALUE_DEFAULT_ARRAY_CAPACITY;
+    static const SizeType kDefaultObjectCapacity = RAPIDJSON_VALUE_DEFAULT_OBJECT_CAPACITY;
 
     struct Flag {
 #if RAPIDJSON_48BITPOINTER_OPTIMIZATION
@@ -2150,7 +2192,7 @@ typedef GenericValue<UTF8<> > Value;
     \tparam StackAllocator Allocator for allocating memory for stack during parsing.
     \warning Although GenericDocument inherits from GenericValue, the API does \b not provide any virtual functions, especially no virtual destructor.  To avoid memory leaks, do not \c delete a GenericDocument object via a pointer to a GenericValue.
 */
-template <typename Encoding, typename Allocator = MemoryPoolAllocator<>, typename StackAllocator = CrtAllocator>
+template <typename Encoding, typename Allocator = RAPIDJSON_DEFAULT_ALLOCATOR, typename StackAllocator = RAPIDJSON_DEFAULT_STACK_ALLOCATOR >
 class GenericDocument : public GenericValue<Encoding, Allocator> {
 public:
     typedef typename Encoding::Ch Ch;                       //!< Character type derived from Encoding.
@@ -2535,6 +2577,7 @@ class GenericDocument : public GenericValue<Encoding, Allocator> {
 //! GenericDocument with UTF8 encoding
 typedef GenericDocument<UTF8<> > Document;
 
+
 //! Helper class for accessing Value of array type.
 /*!
     Instance of this helper class is obtained by \c GenericValue::GetArray().