perf: skip next block headers if they are already inserted.

[ielashi]

As the Bitcoin canister is syncing, it can encounter the following scenario frequently if it's falling far behind:

// Initial state
unstable_state = {
  blocks: {A, B, C};
  next_block_headers: {D, E, F}
}

// Calls get_successors, and gets the following response:
get_successors_response {
  blocks: {D},
  next_block_headers: {E, F}
}

In this scenario, the block headers E and F are processed twice, and that is relatively expensive.

This commit adds logic to check if a block header has already been added to the unstable state and, if yes, skips it. A benchmark has been added to prevent this from regressing.

Before the optimization:

insert_block_headers_multiple_times
                        time:   [118.21 B Instructions 118.21 B Instructions 118.21 B Instructions]

After the optimization:

insert_block_headers_multiple_times
                        time:   [12.055 B Instructions 12.055 B Instructions 12.055 B Instructions]
                        change: [-89.802% -89.802% -89.802%] (p = 0.00 < 0.05)
                        Performance has improved.