index: store pointer to engine

Currently, index iterator allocation and initialization are separated.
This is done in order to speed up iterator creation when the caller
does not yield: there's 'position' method, which returns a preallocated
iterator and can be used instead of costly 'alloc'; the iterator
returned by this method needs to be initialized just like an iterator
allocated normally, via 'alloc'. This looks ugly, because 'position'
is engine-dependent, e.g. it can't be used in case of vinyl, because
vinyl yields internally.

We can get rid of the above-mentioned 'position' hack by simply using a
mempool for allocating iterators. To do that, we need to access engine
from index, so this patch adds a reference to struct engine to struct
index.