ReplicatedPG: fix stat uses to use the correct shard

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG::fill_in_copy_get, use getattr_maybe_cache

Also, add user_only flag to getattr_maybe_cache and cleanup
the other user.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: ensure that ec appends are aligned

When not writing out the last chunk, _write_copy_chunk
will trim the chunk to a stripe boundary for ec pools
and adjust cursor.data_offset to compensate.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ECBackend: handle short reads

async reads may have a length past the end of the object.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: assert no omap for ec pool in _write_copy_chunk and skip

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