erasure-code: locally repairable code plugin

Recursively apply erasure code techniques so that recovering from the
loss of some chunks only require a subset of the available chunks, most
of the time.

The implementation is documented in
doc/rados/operations/erasure-code-lrc.rst

http://tracker.ceph.com/issues/7238 Fixes: #7238

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