TODO: do not use fil_* functions for redo log files.

log_t::checkpoint_lock: remove this lock which was used to wait for
async I/O completion.

checkpoint_lock_key
checkpoint_lock: remove now unneeded globals

log_write_checkpoint_info(): remove sync argument because all checkpoint
writes are synchronous now

log_write_checkpoint_info(): remove sync argument

log_group_checkpoint(): merge with the only caller

log_complete_checkpoint(): merge with the only caller

log_t::complete_checkpoint(): remove by merging with the only caller.