Incremental backup extremly slow

Hello

Taking incremental backup like: `mariabackup --backup --incremental-lsn=xx` is way slower than taking a full backup, something like 20x+ as slow.

I did a bit of debugging, and noticed that it spent way longer in xtrabackup_copy_datafile, when doing inc backup (~ 0.02 vs ~ 0.001 (full backup)).

After a bit more trail-and-error, I found that it spent most of the time in a `memset` line. This line: https://github.com/MariaDB/server/blob/da05d0276a0569341c8bb41365dc7b05f9c4ddb7/extra/mariabackup/write_filt.cc#L80

I found that by adding the following print statement to the file: https://github.com/klausenbusk/server/commit/afcdb9128927e714ee0e3bcf5c14fea2f56855e0
Which gave me:
inc: 0.000001
inc: 0.000004
inc: 0.027387
inc: 0.027435

Ideally the code should only write delta file, when something has changed. At the moment it write a lot of "dummy" delta files, which only contain the header.

Regards Kristian Klausen