Properly benchmark on_initize hook weight

[xlc]

Observed it is not possible to include some transaction on some block, which doesn't really make sense.

Need to do proper benchmark and return the real weight.

https://github.com/paritytech/substrate/blob/c0793b58c509f352b6006e402350ac9ad8017823/frame/session/src/lib.rs#L571

https://github.com/paritytech/substrate/blob/c0793b58c509f352b6006e402350ac9ad8017823/frame/democracy/src/lib.rs#L1625-L1626

[bkchr]

CC @ggwpez

[ggwpez]

Looks like the session weight was a hack from here paritytech/substrate#5738

as suggested by shawn new_session take full block to make it easier, if in the future it is wanted we can improve on this.

Other one from paritytech/substrate#5828 is also old code rotting…

[bkchr]

#4573 explains another issue that appeared because of this.