Merge branch 'blueprint-multipart-primary-key'

Conflicts:
	mod/box/index.h
	mod/box/index.m
	mod/box/tree.m
	test/box/lua.result
	test/box/lua.test
	test/box_big/tarantool.cfg

As part of the merge, revers int -> u32
type change for key_cardinality, done in the
blueprint-multipart-primary-key branch (didn't
notice during code review, but it breaks
formatting in errcode.h).

Merge new and old tests in box_big
manually.