Some improvements for ShipStat collection

[gakada]

This should address cases when records for some ship forms don't converge to a single pair of lv1/lv99 values.

• last_timestamp can be used when devs update stats, to filter out old records (_id timestamp is the first timestamp, since update doesn't update it).
• count can be used to filter out unpopular records (e.g., bugs that can prevent from finding lv1/lv99 values).
• All reported fields are used as keys, so each record is a unique configuration/event (observed stats for a given ship at a given level) with information about first observation time, last observation time, and the total number of observations for that configuration.

To give some examples, current problems finding lv1/lv99 values:

• Non-convergent records, most likely bugs (like single conflicting record for Fusou from ~1 year ago, count can be useful) or updates (like Hiei from today's update, last_timestamp can be useful):

Fusou Kai Ni asw : poi: ?, wctf: 8/28
Hiei Kai Ni los : poi: ?, wctf: 16/52
Hiryuu Kai Ni evasion : poi: ?, wctf: 40/83
Kasumi Kai Ni B los : poi: ?, wctf: 15/56
Maikaze Kai asw : poi: ?, wctf: 27/72
Yamakaze Kai asw : poi: ?, wctf: 30/73

• Different from WCTF, timestamp info can be useful:

Asashimo asw : poi: 35/70, wctf: 35/64
Gotland asw : poi: 36/58, wctf: 35/58
Hamanami Kai asw : poi: 28/65, wctf: 29/65
Saratoga Kai los : poi: 48/80, wctf: 52/80

• Not enough data here (not addressed by this PR, should be fixed on the client side by submitting more reports from other endpoints):

Maruyu evasion : poi: 8~9/19, wctf: 9/19
Maruyu los : poi: 0~1/9, wctf: 1/9

[gakada]

@KagamiChan any comment on this? I just tried a migration test, seems like no migration is required, just restarting the server, old database records will start updating with new info, new records will be added, etc. No updates for the client either.

[KagamiChan]

sorry, completely missed this PR because I was traveling during that period.

This PR looks good to me, cool.

P.S. the ship stat, we created a script last year to verify the data, maybe you'll be interested: https://github.com/KagamiChan/wctf-db-ship-stat-verify