osd: erasure code benchmark workunit

Display benchmark results for the default erasure code plugins, in a tab
separated CSV file. The first two column contain the amount of bytes
that were coded or decoded, for a given combination of parameters
displayed in the following fields.

seconds	bytes	plugin	k	m	work.	iter.	size	eras.	param.
1.2	1024	example	2	1	encode	1	1024	0
0.5	1024	example	2	1	decode	1	1024	1

It can be used as input for a human readable report. It is also intented
to be used to show if a given version of an erasure code plugin performs
better than another.

Signed-off-by: Loic Dachary <loic@dachary.org>