Optionally use libpmem for InnoDB redo log writing.

When server is built -DWITH_PMEM=ON InnoDB tries to detect
that redo log is located on persistent memory storage and
uses faster file access method.

When server is built with -DWITH_PMEM=OFF preprocessor is
used to ensure that no slowdown will present due to allocations
and virtual function calls. So, we don't slow down server
in a common case.

mapped_file_t: an map file, unmap file and returns mapped memory buffer

file_io: abstraction around memory mapped files and file descriptors.
Allows writing, reading and flushing to files.

file_io::writes_are_durable(): notable method of a class.
When it returns true writes are flushed immediately.

file_os_io: file descriptor based file access. Depends on a global state
like srv_read_only_mode

file_pmem_io: file access via libpmem

This is a collaboration work with Sergey Vojtovich