Update default buffer size

[thinkingfish]

To subtract 16 bytes (metadata overhead for malloc) from the current default size, which is exactly 16KB

[kevyang]

Would we also want to update dbuf so that one double gives 32KB - 16 bytes, 2 doubles give 64KB - 16, etc? Or do you think it's acceptable to simply double them?

[kevyang]

Probably the former, as it would make the best fit in a whole number of pages, correct?

[kevyang]

maybe this is a silly question, but would it be better to use mmap to allocate buffer in this case, so we can also have the buffer page aligned?