erasure-code: Add parity delta write optimization to ISA

erasure-code: Add parity delta write optimization to ISA

This commit adds FLAG_EC_PLUGIN_PARITY_DELTA_OPTIMIZATION to the list of
optimizations supported by the ISA plugin, and encode_delta and
apply_delta functions to ISA. Stubs for these functions are added to
the other plugins.
Two new tests have been added to TestErasureCodePlugins.cc to test the
encode_delta and apply_delta functions. The first new test updates a
single parity chunk using a single delta. The second new test updates
all parity chunks with a delta for every data chunk.

Signed-off-by: Jamie Pryde <jamiepry@uk.ibm.com>
Signed-off-by: Alex Ainscow <aainscow@uk.ibm.com>