PGBackend: make rollback_append virtual

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

Signed-off-by: Samuel Just <sam.just@inktank.com>
DNM: ReplicatedPG: restrict WRITE for erasure pools to aligned offsets

heardcoded alignment for testing
librados: add calls to determine required write alignment

Signed-off-by: Samuel Just <sam.just@inktank.com>
TestRados: send aligned appends on erasure pools

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