[t:4261], set minimum bulk fetch buffer size to 0 on main line

git-svn-id: file:///svn/mysql/tokudb-engine/tokudb-engine@37767 c7de825b-a66e-492c-adef-691d508d4ae1

storage/tokudb/hatoku_hton.cc

@@ -133,7 +133,7 @@ static MYSQL_THDVAR_UINT(read_buf_size,
   NULL, 
   NULL, 
   128*1024, // default
-  4096,  // min
+  0,  // min
   1*1024*1024,   // max
   1      // blocksize???
   );