Previously, the "dynamic batch size" strategy for the vectorized
engine's batch allocator worked the same in every situation: batches
would start at size 1, then double on each re-allocation, until they hit
their maximum size of 1024.

Now, to improve performance for scans that return a number of rows
somewhere in between 1 and 1024, we pass in the optimizer's best guess
of the number of rows that the scan will produce all the way down into
the TableReader. That guess is used as the initial size of the batch if
it's less than 1024.

Release note (performance improvement): improve the performance for the
vectorized engine when scanning fewer than 1024 rows at a time.