IF: make constant members of block_state and block_header_state const. Make block_header_state a member. of block_state.

[greg7mdp]

In addition to documenting which members don't change, it will allow to add a std::mutex to pending_qc , which will allow safe access from multiple net_plugin threads for aggregating votes.

It will require (at least) making block_header_state inside assembled_block_if a unique_ptr, because const members are not movable.

Also, probably less importantly, make block_header_state a member of block_state instead of inheriting from it.