This commit refactors the row container infrastructure in order to make
it possible to read from a single container concurrently (via separate
iterators). It required moving some of the scratch space from the
container directly into the iterators (so that the iterators don't share
anything between each other) as well as making a similar adjustment to
the `pebbleMapIterator`. As a result, it is now possible to safely
concurrently iterate over all types of row containers; however, the
iterators need to be created and closed under mutex protection (which
seems like an acceptable limitation). This commit also makes a minor
change to make the container a named parameter inside of the iterator
(which makes it easier to see when an iterator is touching the
container).

The ability to read from the same row container (after all rows have
been added to it) concurrently will be utilized by the follow-up commit
in order to support validation of multiple FK and UNIQUE checks in
parallel.

Additionally, this commit improves `RowIterator` interface a bit by
renaming `Row` to `EncRow` (since the method returns `EncDatumRow`s) and
introducing a new `Row` implementation which returns `tree.Datums`. The
new method is now utilized in the `sql.rowContainerIterator` which
removes some of the unnecessary conversions between different row types.

Release note: None