overcounting of memory in first/last.

[ashdnazg]

Describe the bug

When aggregating first/last list over a column of lists, the first/last
accumulators hold the necessary scalar value as is, which points to the
list in the original input buffer.

This results in two issues:

1. We prevent the deallocation of the input arrays which might be
   significantly larger than the single value we want to hold.

2. During aggreagtion with groups, many accumulators receive slices of the
   same input buffer, resulting in all held values pointing to this buffer.
   Then, when calculating the size of all accumulators we count the buffer
   multiple times, since each accumulator considers it to be part of its own
   allocation.

While 1 can be tolerated, 2 can easily result in an OOM for a small input with a few thousand groups.