greenplum->copy_records_to_table support needed

• asyncpg version:0.17.0
• Greenplum version:5.7
• Python version:3.7
• Platform:Windows 10/windows 10 WSL
• Do you use pgbouncer?:no
• Did you install asyncpg with pip?:yes
• If you built asyncpg locally, which version of Cython did you use?:n/a
• Can the issue be reproduced under both asyncio and
  uvloop?:n/a

try to run copy_records_to_table against greenplum 5.7 which report error as:
File "D:\Anaconda3\envs\DataLightFire37\lib\site-packages\asyncpg\connection.py", line 848, in _copy_in_records
copy_stmt, None, None, records, intro_stmt, timeout)
File "asyncpg\protocol\protocol.pyx", line 504, in copy_in
asyncpg.exceptions.PostgresSyntaxError: syntax error at or near "("

Try to run those against PostgreSQL 9.2. It passed.
Also, execute and executemany work fine with Greenplum.

首先在 Greenplum 5.7 和 PostgreSQL 9.2 上复现 copy_records_to_table，然后检查报告失败的 connection.py 第 848 行和 protocol.pyx 第 504 行。比较生成的 COPY 语句，并验证在 Greenplum 上复制记录能够正常工作，同时不会导致 PostgreSQL 行为回归。