Bug fix: don't send RowDescription message with each spooled batch of rows

[fulghum]

Ensure that RowDescription messages are sent only once, at the beginning of query results. Sending multiple times, at the start of each batch of spooled rows, breaks some clients, such as psycopg2.

Note that this wasn't causing a problem in the other clients we use for testing. To properly regression test this, we should follow up with adding a test suite for psycopg2.

Related to #2143

[zachmu]

LGTM!

[github-actions]

	Main	PR
covering_index_scan_postgres	561.38/s	576.43/s	+2.6%
index_join_postgres	115.79/s	122.89/s	+6.1%
index_join_scan_postgres	178.42/s	183.49/s	+2.8%
index_scan_postgres	11.20/s	11.48/s	+2.5%
oltp_point_select	2262.10/s	2388.13/s	+5.5%
oltp_read_only	1687.87/s	1726.90/s	+2.3%
select_random_points	104.38/s	109.60/s	+5.0%
select_random_ranges	417.20/s	440.36/s	+5.5%
table_scan_postgres	10.64/s	11.04/s	+3.7%
types_table_scan_postgres	4.97/s	5.09/s	+2.4%

[github-actions]

	Main	PR
Total	42090	42090
Successful	17563	17563
Failures	24527	24527
Partial Successes1	5582	5582

	Main	PR
Successful	41.7273%	41.7273%
Failures	58.2727%	58.2727%

Footnotes

1. These are tests that we're marking as Successful, however they do not match the expected output in some way. This is due to small differences, such as different wording on the error messages, or the column names being incorrect while the data itself is correct. ↩