fix stmtcache invalidation

This patch fixes jackc/pgx#841. The meat of the fix lives
in [a PR to the pgconn repo][1]. This change just checks
for errors after executing a prepared statement and informs
the underlying stmtcache about them so that it can properly
clean up. We don't try to get fancy with retries or anything
like that, just return the error and allow the application to handle it.

I had to make [some][1] [changes][2] to to the jackc/pgconn package as well
as this package.

Fixes #841

[1]: https://github.com/jackc/pgconn/pull/56
[2]: https://github.com/jackc/pgconn/pull/55

rows.go

@@ -106,6 +106,7 @@ type connRows struct {
 	sql        string
 	args       []interface{}
 	closed     bool
+	conn       *Conn
 
 	resultReader      *pgconn.ResultReader
 	multiResultReader *pgconn.MultiResultReader
@@ -145,8 +146,13 @@ func (rows *connRows) Close() {
 				endTime := time.Now()
 				rows.logger.log(rows.ctx, LogLevelInfo, "Query", map[string]interface{}{"sql": rows.sql, "args": logQueryArgs(rows.args), "time": endTime.Sub(rows.startTime), "rowCount": rows.rowCount})
 			}
-		} else if rows.logger.shouldLog(LogLevelError) {
-			rows.logger.log(rows.ctx, LogLevelError, "Query", map[string]interface{}{"err": rows.err, "sql": rows.sql, "args": logQueryArgs(rows.args)})
+		} else {
+			if rows.logger.shouldLog(LogLevelError) {
+				rows.logger.log(rows.ctx, LogLevelError, "Query", map[string]interface{}{"err": rows.err, "sql": rows.sql, "args": logQueryArgs(rows.args)})
+			}
+			if rows.err != nil {
+				rows.conn.stmtcache.StatementErrored(rows.sql, rows.err)
+			}
 		}
 	}
 }