promote both db and cf stats and turn enums into enum class

[gimre-xymcity]

This is proposed fix for #4946 as stated in comment

this fix probably has invalid formatting and might have few issues, but it seems to be compiling,
(I haven't checked ROCKSDB_LITE ifdef branch though).
The fix consists of fairly simple changes:

1. promotes InternalCFStatsType and InternalDBStatsType to top rocksdb namespace
2. turns both enums into enum classes
3. fixes usage
   (this is on top of 6.2.fb branch, but it should be fairly trivial to rebase if needed)

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[siying]

We generally don't accept pull requests to release branches. Everything must go to master first and the release manager will backport it. I believe master has an alternative fix: #5779 .

@gimer if we release 6.3 in a week or so with this fixed, do you still want 6.2 to be fixed?

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[gimre-xymcity]

We generally don't accept pull requests to release branches. Everything must go to master first and the release manager will backport it.

ok, will know in future.

I believe master has an alternative fix: #5779 .

Is there any style/convention guide anywhere?

if we release 6.3 in a week or so with this fixed, do you still want 6.2 to be fixed?

not necessarily.

[gimre-xymcity]

@siying just to confirm, the fix from adbc25a isn't available in 6.3 branch yet, correct?

[siying]

@gimer you are right. Not only it is not in 6.3 branch, the coming 6.4 won't have it either.