vinyl: align statements and bps tree extents

Vinyl tuples (vy_stmt) in 0 level of LSM tree are stored in
lsregion. They were allocated using lsregion_alloc(), which does
not align its results, and is good only for byte arrays.

As a result, vy_stmt object addresses in 0 LSM level were not
aligned. Unaligned memory access is slower, and may even crash on
some platforms.

Besides, even aligned allocations couldn't help upserts in 0 level
of the LSM tree, because upsert vy_stmt objects had 1 byte prefix
to count merged upserts stored in this statement. This 1 byte
prefix ruined all the alignment. Now the upsert counter is also
aligned, the same as vy_stmt. Note, it does not consume
significantly more memory, since is used only for vinyl and only
for upserts, stored in 0 level of the LSM tree.

The same about BPS tree extents. LSM 0 level is a BPS tree, whose
blocks are allocated on lsregion. The extents are used as pointer
arrays inside the tree, so they need alignof(void *) alignment.

The mentioned unaligned accesses were revealed by clang undefined
behaviour sanitizer, and are fixed by this patch.

Part of #4609