Call a correct connection_buf_add* function based on compress_state of dir_connection_t
In #21377 (moved), we discovered that it is easy to set the compression state on a connection, but add uncompressed data to that connection.
We should log a bug warning when:
- conn->compress_state is not NULL, and connection_buf_add() is called
- conn->compress_state is NULL, and connection_buf_add_compress() is called
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