Quotas: review fixes.

src/box/lua/slab.cc

src/box/tree_index.cc

src/box/tuple.cc

src/box/tuple.h

src/lib/small/quota.h

src/lib/small/slab_arena.c

src/memory.cc

src/memory.h

test/unit/mempool.c

test/unit/quota.cc

test/unit/region.c

test/unit/slab_arena.c

test/unit/slab_arena.result

test/unit/small_alloc.c