ErasureCode: example implementation : K=2 M=1

ErasureCode: example implementation : K=2 M=1

An erasure code implementation designed for tests. Although it is fully
functional and could be used on actual data, it is mainly provided for
testing purposes. It splits data in two, computes an XOR parity and
can sustain the loss of one chunk.

The constructor will usleep(3) for parameters["usleep"] microseconds
so that the caller can create race conditions.

http://tracker.ceph.com/issues/5878 refs #5878

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