Optimize ranking calculation

[inada-s]

As the number of battle records has increased, it will take longer to calculate.
I believe it can be speeded up by saving the calculated state in a table.

[vkedwardli]

Maybe it's also a time to review the ranking logic? Sorting by "total winning count" seems to be returning the Most Hardworking Players instead of their true skills.

so may be something like https://github.com/mafredri/go-trueskill ?

[inada-s]

#240