Align the data block sizes of pg_dump's various compression modes.

Align the data block sizes of pg_dump's various compression modes.

After commit fe8192a95, compress_zstd.c tends to produce data block
sizes around 128K, and we don't really have any control over that
unless we want to overrule ZSTD_CStreamOutSize().  Which seems like
a bad idea.  But let's try to align the other compression modes to
produce block sizes roughly comparable to that, so that pg_restore's
skip-data performance isn't enormously different for different modes.

gzip compression can be brought in line simply by setting
DEFAULT_IO_BUFFER_SIZE = 128K, which this patch does.  That
increases some unrelated buffer sizes, but none of them seem
problematic for modern platforms.

lz4's idea of appropriate block size is highly nonlinear:
if we just increase DEFAULT_IO_BUFFER_SIZE then the output
blocks end up around 200K.  I found that adjusting the slop
factor in LZ4State_compression_init was a not-too-ugly way
of bringing that number roughly into line.

With compress = none you get data blocks the same sizes as the
table rows, which seems potentially problematic for narrow tables.
Introduce a layer of buffering to make that case match the others.

Comments in compress_io.h and 002_pg_dump.pl suggest that if
we increase DEFAULT_IO_BUFFER_SIZE then we need to increase the
amount of data fed through the tests in order to improve coverage.
I've not done that here, leaving it for a separate patch.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/3515357.1760128017@sss.pgh.pa.us