Commit 32a9c0bd introduced connection retry logic into postgres_fdw.
Previously it used goto statement for retry. This commit gets rid of that
goto from the logic to make the code simpler and easier-to-read.

When getting out of PG_CATCH() for the retry, the error state should be
cleaned up and the memory context should be reset. But commit 32a9c0bd
forgot to do that. This commit also fixes this bug.

Previously only PQstatus()==CONNECTION_BAD was verified to detect
connection failure. But this could cause false detection in the case where
any error other than connection failure (e.g., out-of-memory) was thrown
after a broken connection was detected in libpq and CONNECTION_BAD is set.
To fix this issue, this commit changes the logic so that it also checks
the error's sqlstate is ERRCODE_CONNECTION_FAILURE.

Author: Fujii Masao
Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/2943611.1602375376@sss.pgh.pa.us