vinyl: fix latency accounting for point lookups

We don't account latency of point lookups, neither we emit a warning if
a point lookup took > too_long_threshold. To fix this, let's move the
too_long_threshold configuration parameter from vy_env to vy_index_env
and check it in both vy_read_iterator_next() and vy_point_lookup().
This will also allow us not to pass too_long_threshold (or vy_env for
that matter) all the way down to read iterator initialization.