vinyl: pass read view to vy_index_get explicitly

Currently, if tx is not NULL, tx->read_view is used, otherwise the
global read view is used. For the sake of #2534 (read view for read only
autocommit statements), we will need to pass an arbitrary read view to
this function. So let's add the corresponding argument. This also allows
us to drop env from the argument list.

While we are at it, let's also inline vy_index_full_by_stmt() as it is a
trivial wrapper around vy_index_get().

Needed for #2534