[Patch] PQconnectPoll() is blocked if target_session_attrs is read-write

Hi

# I rewrote my previous mail.

PQconnectPoll() is used as method for asynchronous using externally or internally.
If a caller confirms a socket ready for writing or reading that is
requested by return value of previous PQconnectPoll(), next PQconnectPoll()
must not be blocked. But if the caller specifies target_session_attrs to
'read-write', PQconnectPoll() may be blocked.

Detail:
If target_session_attrs is set to read-write, PQconnectPoll() calls
PQsendQuery("SHOW transaction_read_only") althogh previous return value was
PGRES_POLLING_READING not WRITING.
In result, PQsendQuery() may be blocked in pqsecure_raw_write().

I attach a patch.

Regards
Ryo Matsumura