vinyl: take into account primary key lookup in latency accounting

Currently, latency accounting and warning lives in vy_point_lookup and
vy_read_iterator_next. As a result, we don't take into account full by
partial tuple lookup in it while it can take quite a while, especially
if there are lots of deferred DELETE statements we have to skip. So this
patch moves latency accounting to the upper level, namely to vy_get and
vinyl_iterator_{primary,secondary}_next.

Note, as a side effect, now we always print full tuples to the log on
"too long" warning. Besides, we strip LSN and statement type as those
don't make much sense anymore.