实现mmap参数化

[WangTingZheng]

老是改kDefaultMmapLimit不太好，这个是在编译期就计算出来的。

[WangTingZheng]

env_posix_test_helper.h里的EnvPosixTestHelper可以用一下：

// A helper for the POSIX Env to facilitate testing.
class EnvPosixTestHelper {
 private:
  friend class EnvPosixTest;

  // Set the maximum number of read-only files that will be opened.
  // Must be called before creating an Env.
  static void SetReadOnlyFDLimit(int limit);

  // Set the maximum number of read-only files that will be mapped via mmap.
  // Must be called before creating an Env.
  static void SetReadOnlyMMapLimit(int limit);
};

[WangTingZheng]

发现很难，必须在Env创建前就需要改值，可以从NewRandomAccessFile入手。