avoid unnecessary hash lookups

[GrapeBaBa]

loop over 0..numValidators() (real network the number is big), doing a HashMap .get() for each. Most validators won't have a tracker entry, so most iterations are wasted hash lookups. Even those that do exist get an unnecessary .put() write-back

https://github.com/blockblaz/zeam/blob/main/pkgs/node/src/forkchoice.zig#L891-L895

https://github.com/blockblaz/zeam/blob/main/pkgs/node/src/forkchoice.zig#L987-L1004