Couchstore: Change the default blocksize

Problem
The default block used by couchstore is 4KiB. Cloud environment and SSD recommend using a larger blocksize to reduce IOPs and increase performance

Suggestion
Ideally couchstore would detect the underlying blocksize and match it, so Couchbase would be block aligned through the stack. That might be tricky to do, being able to set the blocksize might be a cheap win.

Pro
Reduces the number of IOP need to perform an operation, improves overall performance
Works better with documents over 4KiB
Reduce the overhead for the index blocks

Con
Reduce the number of blocks in the filesystem cache
For smaller documents might make things worst, the file system cache might be more fragmented.

Notes
I suggest this as it might be a cheap easy win.