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 KB
that were coded or decoded, for a given combination of parameters
displayed in the following fields.

seconds	KB	plugin	k	m	work.	iter.	size	eras.
1.2	10	example	2	1	encode	10	1024	0
0.5	10	example	2	1	decode	10	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.

The last column ( not shown above for brievety ) is the exact command
that was run to produce the result so it can be copy / pasted to
reproduce them or to profile.

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