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ARROW-6141: [C++] Enable memory-mapping a file region

This patch adds an Open() method for MemoryMappedFile() with
length and offset params. In this way user can memory map a file
region just like mmap(). The new API is:

* MemoryMappedFile::Open(path, mode, length, offset, &mmap)

The original API is still available. Calling the original API
will memory map the whole file:

* MemoryMappedFile::Open(path, mode, &mmap)

A new field map_len_ is added in MemoryMappedFile::MemoryMap to
track the real memory map length.

Also MemoryMappedFile::Read()/ReadAt()/Write()/WriteAt() are changed
to check the memory map length if it's a region based memory map.

Note the MemoryMappedFile::Resize() is not supported if it's a
region based memory map.

Signed-off-by: Yuan Zhou <yuan.zhou@intel.com>

Closes #5101 from zhouyuan/wip_6141 and squashes the following commits:

030994bcc <Yuan Zhou> ARROW-6141:  Enable memory-mapping a file region

Authored-by: Yuan Zhou <yuan.zhou@intel.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>