osd: erasure code benchmark tool

Implement the ceph_erasure_code_benchmark utility to:

* load an erasure code plugin
* loop over the encode function using the parameters from the command
  line
* print the number of bytes encoded and the time to process

For instance:

    $ ceph_erasure_code_benchmark \
       --plugin jerasure \
       --parameter erasure-code-directory=.libs \
       --parameter erasure-code-technique=reed_sol_van \
       --parameter erasure-code-k=2 \
       --parameter erasure-code-m=2 \
       --iterations 1000
    0.964759  1048576000

shows 1GB is encoded in 1second.

Signed-off-by: Loic Dachary <loic@dachary.org>