log_t: Define buf_size, max_buf_free as 32-bit and next_checkpoint_no
as byte (we only need a bit) and rearrange some data members,
so that on AMD64 we can fit log_sys.latch and log_sys.log in
the same 64-byte cache line.

mtr_t::commit_log(), mtr_t::commit_logger: A part of mtr_t::commit()
split into a separate function, so that we will not unnecessarily invoke
log_sys.get_write_target() when running on a memory-mapped log file,
or log_sys.is_pmem().

Reviewed by: Vladislav Vaintroub
Tested by: Matthias Leich