MB-7629: Make btree chunk size configurable

Change-Id: Ia24b1f9ea1579c6c06b5f424b2da00f00c4eb943
Reviewed-on: http://review.couchbase.org/25095
Tested-by: Fulu Li <fulu@couchbase.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>