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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