arrow

ARROW-6613: [C++] Minimize usage of boost::filesystem

Allow building Arrow core (without tests, without filesystem layer) without boost dependencies.

The `minimal_build` example produces something like this:
```
# size  /usr/local/lib/libarrow.so
   text	   data	    bss	    dec	    hex	filename
4001919	  84992	   1712	4088623	 3e632f	/usr/local/lib/libarrow.so
# ldd  /usr/local/lib/libarrow.so
	linux-vdso.so.1 (0x00007fff45f40000)
	libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f7089b08000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f708976a000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f7089552000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f7089333000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f7088f42000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f708a478000)
```

Closes #5545 from pitrou/ARROW-6613-boost-fs-optional and squashes the following commits:

4757240b6 <Antoine Pitrou> Address review comments
6ac957778 <Antoine Pitrou> ARROW-6613:  Minimize usage of boost::filesystem

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>