index: implement generic versions of min(), max(), and count()

The primary reason for these methods to be implemented differently
for memtx and vinyl was the 'position' optimization exploited by
the memtx engine: since selects from memtx do not yield, we could
use a preallocated iterator there.

Now, as the 'position' optimization became redundant and was
removed due to the switch to memory pools for iterator allocations,
the only idiosyncrasy left in the memtx implementation is the count()
optimization: count() falls back on size() for ITER_ALL. Since this
optimization consists of just a few lines of code, we don't really
need memtx_index_count() co-used by all memtx index implementations:
we can implement it in each memtx index separately.

That being said, let us:
 - implement generic versions of min(), max(), and count();
 - make vinyl, memtx, and sysview engines use generic versions of
   the above-mentioned methods if appropriate;
 - Remove memtx_index.[hc]

As a side-effect, this patch enables min(), max(), and count() in
the sysview engine, but that is not bad considering that this engine
implements general-purpose iterator for its indexes.