Re-work posix_fadvise, sync_file_range buffer cache changes.

Put per-object os_cache_max configuration back in, but implement it in
the block manager layer.  Disallow if direct_io is configured, or if the
system doesn't support posix_fadvise.

Add per-object os_write_schedule, if configured immediately schedule
any writes.  Disallow if direct_io is configured, or if the system
doesn't support sync_file_range.