I tried to rename a key column in a MergeTree table. It does not work, and 20.4 is very clear about the reason. 
20.5.2 is complaining about non-existing column with the old name, suggesting there is a column with the new name:
```
ALTER TABLE rename_key
    RENAME COLUMN key TO new

Received exception from server (version 20.4.6):
Code: 44. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key key column which is a part of key expression.

Received exception from server (version 20.5.2):
Code: 47. DB::Exception: Received from localhost:9000. 
DB::Exception: Missing columns: 'key' while processing query: 'key', required columns: 'key', source columns: 'value' 'new' 'date'.
```
The message hasn't changed for the partition key:
```
ALTER TABLE rename_key
    RENAME COLUMN date TO new

Received exception from server (version 20.4.6):
Code: 44. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key date column which is a part of key expression.

Received exception from server (version 20.5.2):
Code: 524. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key date column which is a part of key expression.
```
The table:
```
CREATE TABLE rename_key
(
    `date` Date,
    `key` UInt64,
    `value` String
)
ENGINE = MergeTree()
PARTITION BY date
ORDER BY key
```

{
    auto columns = metadata.columns;
    for (size_t i = 0; i < size(); ++i)
    {
        auto & command = (*this)[i];
        bool has_column = columns.has(command.column_name) || columns.hasNested(command.column_name);
        if (command.type == AlterCommand::MODIFY_COLUMN)
        {
            if (!has_column && command.if_exists)
                command.ignore = true;
            if (has_column && command.data_type)
            {
                auto column_from_table = columns.get(command.column_name);
                if (!command.default_expression && column_from_table.default_desc.expression)
                {
                    command.default_kind = column_from_table.default_desc.kind;
                    command.default_expression = column_from_table.default_desc.expression;
                }
            }
        }
        else if (command.type == AlterCommand::ADD_COLUMN)
        {
            if (has_column && command.if_not_exists)
                command.ignore = true;
        }
        else if (command.type == AlterCommand::DROP_COLUMN
                || command.type == AlterCommand::COMMENT_COLUMN
                || command.type == AlterCommand::RENAME_COLUMN)
        {