ECBackend: flesh out ECBackend implementation

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/: instantiate the right backend based on pool

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGBackend: add get_shard_(info|missing)(pg_shard_t) helpers

ECBackend's primary specific logic mostly win't treat the
primary shard specially, so it'll be handy to have primary
agnostic helpers for these structures.

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