src/osd/: add ECBackend stubs

Signed-off-by: Samuel Just <sam.just@inktank.com>
messages,osd: add EC messages and associated types

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/ECUtil: add ec encode/decode util helpers

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/ECTransaction: add type encapsulating EC transaction

Signed-off-by: Samuel Just <sam.just@inktank.com>
ECBackend: start fleshing out ECBackend client IO path

Signed-off-by: Samuel Just <sam.just@inktank.com>
ECBackend: start fleshing out ECBackend recovery

Signed-off-by: Samuel Just <sam.just@inktank.com>