greenplum->copy_records_to_table support needed

Nadie ha tomado este issue todavía.

* **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](https://github.com/magicstack/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.

Start by reproducing copy_records_to_table against Greenplum 5.7 and PostgreSQL 9.2, then inspect connection.py line 848 and protocol.pyx line 504 where the failure is reported. Compare the generated COPY statements and verify that record copying works on Greenplum without regressing PostgreSQL behavior.

Escrito por el modelo de indexación a partir del texto del issue.