27341: importccl: add oversample option to configure oversampling r=mjibson a=mjibson

On clusters with many nodes and smallish disks, doing an IMPORT that is
within an order of magnitude of the free space of the disks can lead to
disk fullness. This happens because the sampling algorithm (by design)
has a relatively high standard deviation in its error rate. We currently
target split points at a few hundred megs, but the standard deviation
on sampling means that a single node could easily be a few of those
away from the target mean, resulting in overscheduling data to a node
during shuffle.

Introduce an oversample option that can be set to some higher number. This
reduces the standard deviation of the error, resulting in each node having
more similar portion of the data, but does not have a major impact on
the rest of the performance.

Release note (sql change): Add an `oversample` WITH option to IMPORT to
decrease variance in data distributionduring processing.

27345: importccl: verify number of columns during IMPORT PGDUMP r=mjibson a=mjibson

Also make error messages more consistent between PGDUMP and PGCOPY.

Release note (bug fix): Correctly verify number of COPY columns during
IMPORT PGDUMP.

27438: mkrelease: statically link windows release binaries r=mberhault a=benesch

This got lost in 38899a8. Static linking is necessary to bundle MinGW-only
libraries into the Windows binary. The binary is otherwise only
executable from within a MinGW environment.

Fix #27435.

Release note: None

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At this point I deserve to win an award for most broken refactor.

Co-authored-by: Matt Jibson <matt.jibson@gmail.com>
Co-authored-by: Nikhil Benesch <nikhil.benesch@gmail.com>