Riegeli usage cleanup:

[copybara-service]

Riegeli usage cleanup:

Use riegeli::SharedPtr instead of std::shared_ptr. Do not convert it from
std::unique_ptr: riegeli::SharedPtr always allocates the control block
together with the object, hence this is not supported, and this is more
efficient anyway.

Utilize CTAD more.

Let MaskedReader constructor take riegeli::Reader& instead of
std::unique_ptr<riegeli::Reader>. There is no ownership transfer, the
riegeli::Reader is accessed only during construction.

Use riegeli::SharedBuffer instead of std::shared_ptr<std::string> in
MaskedReader. It has faster refcounting and a smaller overhead. It does not
track the exact size but this is not needed here.

Use defaulted move constructor and move assignment of MaskedReader. They do
the right thing.

Add MaskedReader::Reset() to reset the instance to a state equivalent to a
newly constructed state in-place.

Use std::optional instead of absl::optional.

Use std::optional or storing the value directly instead of std::unique_ptr
when movability is not needed.