Use DefaultQueryExecMode in CopyFrom

CopyFrom had to create a prepared statement to get the OIDs of the data
types that were going to be copied into the table. Every COPY operation
required an extra round trips to retrieve the type information. There
was no way to customize this behavior.

By leveraging the QueryExecMode feature, like in `Conn.Query`, users can
specify if they want to cache the prepared statements, execute
them on every request (like the old behavior), or bypass the prepared
statement relying on the pgtype.Map to get the type information.

The `QueryExecMode` behave exactly like in `Conn.Query` in the way the
data type OIDs are fetched, meaning that:

- `QueryExecModeCacheStatement`: caches the statement.
- `QueryExecModeCacheDescribe`: caches the statement and assumes they do
  not change.
- `QueryExecModeDescribeExec`: gets the statement description on every
  execution. This is like to the old behavior of `CopyFrom`.
- `QueryExecModeExec` and `QueryExecModeSimpleProtocol`: maintain the
  same behavior as before, which is the same as `QueryExecModeDescribeExec`.
  It will keep getting the statement description on every execution

The `QueryExecMode` can only be set via
`ConnConfig.DefaultQueryExecMode`, unlike `Conn.Query` there's no
support for specifying the `QueryExecMode` via optional arguments
in the function signature.

conn.go

@@ -721,43 +721,10 @@ optionLoop:
 	sd, explicitPreparedStatement := c.preparedStatements[sql]
 	if sd != nil || mode == QueryExecModeCacheStatement || mode == QueryExecModeCacheDescribe || mode == QueryExecModeDescribeExec {
 		if sd == nil {
-			switch mode {
-			case QueryExecModeCacheStatement:
-				if c.statementCache == nil {
-					err = errDisabledStatementCache
-					rows.fatal(err)
-					return rows, err
-				}
-				sd = c.statementCache.Get(sql)
-				if sd == nil {
-					sd, err = c.Prepare(ctx, stmtcache.NextStatementName(), sql)
-					if err != nil {
-						rows.fatal(err)
-						return rows, err
-					}
-					c.statementCache.Put(sd)
-				}
-			case QueryExecModeCacheDescribe:
-				if c.descriptionCache == nil {
-					err = errDisabledDescriptionCache
-					rows.fatal(err)
-					return rows, err
-				}
-				sd = c.descriptionCache.Get(sql)
-				if sd == nil {
-					sd, err = c.Prepare(ctx, "", sql)
-					if err != nil {
-						rows.fatal(err)
-						return rows, err
-					}
-					c.descriptionCache.Put(sd)
-				}
-			case QueryExecModeDescribeExec:
-				sd, err = c.Prepare(ctx, "", sql)
-				if err != nil {
-					rows.fatal(err)
-					return rows, err
-				}
+			sd, err = c.getStatementDescription(ctx, mode, sql)
+			if err != nil {
+				rows.fatal(err)
+				return rows, err
 			}
 		}
 
@@ -827,6 +794,48 @@ optionLoop:
 	return rows, rows.err
 }
 
+// getStatementDescription returns the statement description of the sql query
+// according to the given mode.
+//
+// If the mode is one that doesn't require to know the param and result OIDs
+// then nil is returned without error.
+func (c *Conn) getStatementDescription(
+	ctx context.Context,
+	mode QueryExecMode,
+	sql string,
+) (sd *pgconn.StatementDescription, err error) {
+
+	switch mode {
+	case QueryExecModeCacheStatement:
+		if c.statementCache == nil {
+			return nil, errDisabledStatementCache
+		}
+		sd = c.statementCache.Get(sql)
+		if sd == nil {
+			sd, err = c.Prepare(ctx, stmtcache.NextStatementName(), sql)
+			if err != nil {
+				return nil, err
+			}
+			c.statementCache.Put(sd)
+		}
+	case QueryExecModeCacheDescribe:
+		if c.descriptionCache == nil {
+			return nil, errDisabledDescriptionCache
+		}
+		sd = c.descriptionCache.Get(sql)
+		if sd == nil {
+			sd, err = c.Prepare(ctx, "", sql)
+			if err != nil {
+				return nil, err
+			}
+			c.descriptionCache.Put(sd)
+		}
+	case QueryExecModeDescribeExec:
+		return c.Prepare(ctx, "", sql)
+	}
+	return sd, err
+}
+
 // QueryRow is a convenience wrapper over Query. Any error that occurs while
 // querying is deferred until calling Scan on the returned Row. That Row will
 // error with ErrNoRows if no rows are returned.