Added --disk-cache option

This option enables disk cache. If SIZE is 0, the disk cache is
disabled. This feature caches the downloaded data in memory, which
grows to at most SIZE bytes. The cache storage is created for aria2
instance and shared by all downloads. The one advantage of the disk
cache is reduce the disk seek time because the data is written in
larger unit and it is reordered by the offset of the file. If the
underlying file is heavily fragmented it is not the case.

test/TestUtil.h

@@ -4,6 +4,7 @@
 
 #include "SharedHandle.h"
 #include "Cookie.h"
+#include "WrDiskCacheEntry.h"
 
 namespace aria2 {
 
@@ -50,4 +51,8 @@ std::string fileHexDigest
 (const SharedHandle<MessageDigest>& ctx, const std::string& filename);
 #endif // ENABLE_MESSAGE_DIGEST
 
+WrDiskCacheEntry::DataCell* createDataCell(int64_t goff,
+                                           const char* data,
+                                           size_t offset = 0);
+
 } // namespace aria2