vinyl: allocate extents for in-memory trees from a memory pool
vinyl: allocate extents for in-memory trees from a memory pool

We should probably create a separate arena for vinyl or reuse the memtx
arena. For now, allocate from the generic cache.

Closes #1767

mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,mejedi/tarantool,mejedi/tarantool,mejedi/tarantool,rtsisyk/tarantool,rtsisyk/tarantool