osd: conditionally disable dlclose of erasure code plugins

When profiling, tools such as valgrind --tool=callgrind require that the
dynamically loaded libraries are not dlclosed so they can collect usage
information.

The public ErasureCodePluginRegistry::disable_dlclose boolean is introduced
for this purpose.

Signed-off-by: Loic Dachary <loic@dachary.org>
osd: erasure code benchmark tool

Implement the ceph_erasure_code_benchmark utility to:

* load an erasure code plugin

* loop over the encode/decode function using the parameters from the
  command line

* print the number of bytes encoded/decoded and the time to process

When decoding, the random chunks ( --erasures ) are lost on each run.

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 \
       --workload decode \
       --erasures 2 \
       --iterations 1000
    0.964759,1048576000

shows 1GB is decoded in 1second.

It is intended to be used by other scripts to present a human readable
output or detect performance regressions.

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