store: introduce Database::get_with_rc_stripped method

[mina86]

Add get_with_rc_stripped method to the Database trait which strips the
reference count from the returned value.

Previously, Store::get method called get_raw_bytes and then (for
reference counted columns) refcount::get_with_rc_logic to strip the
reference count. With this change, it now calls either get_raw_bytes
or get_with_rc_stripped as needed.

The reason for this addition is to help with cold storage. We don’t
want to keep refcounts in cold storage which means that we would need
to append dummy refcount to get_raw_bytes calls against cold storage.
By having a separate method which returns values without refcount, we
won’t need to worry about this dummy count.

As a side benefit, this removes a memory allocation when reading
a reference counted column. Previosly get_payload method was used
to strip the refcount:

fn get_payload(value: Vec<u8>) -> Option<Vec<u8>> {
    decode_value_with_rc(&value).0.map(|v| v.to_vec())
}

The issue with it is that it takes a Vec as argument, takes a slice of
it and creates a new Vec from it. Now the vector is simply truncated
by strip_refcount function.