`dayjs.tz` is not idempotent with dates in DST when it's not currently DST

**Describe the bug**
Given:
* it is currently Standard Time (i.e. not Daylight Saving Time)
* the date being manipulated is in Daylight Saving Time

When `Dayjs#tz` is called more than once on a `Dayjs` object, the time moves back 1 hour each time `Dayjs#tz` is called.
```js
> dayjs('2020-08-08T00:00:00.000Z').toString()
//=> 'Sat, 08 Aug 2020 00:00:00 GMT'

> dayjs('2020-08-08T00:00:00.000Z').tz('America/Chicago').toString()
//=> 'Fri, 07 Aug 2020 23:00:00 GMT'

> dayjs('2020-08-08T00:00:00.000Z').tz('America/Chicago').tz('America/Chicago').toString()
//=> 'Fri, 07 Aug 2020 22:00:00 GMT'

> dayjs('2020-08-08T00:00:00.000Z').tz('America/Chicago').tz('America/Chicago').tz('America/Chicago').toString()
//=> 'Fri, 07 Aug 2020 21:00:00 GMT'
```

The result is the same regardless of what timezones are used, or whether a timezone arg is passed in at all:
```js
> dayjs('2020-08-08T00:00:00.001Z').tz('UTC').tz().tz('UTC').toString()
//=> 'Fri, 07 Aug 2020 21:00:00 GMT'

> dayjs('2020-08-08T00:00:00.001Z').tz('America/Chicago').tz('UTC').tz().toString()
//=> 'Fri, 07 Aug 2020 21:00:00 GMT'

> dayjs('2020-08-08T00:00:00.001Z').tz().tz().tz().toString()
//=> 'Fri, 07 Aug 2020 21:00:00 GMT'
```

It's February and I'm in Chicago, so I've only been able to confirm this issue occurs during CST with a date in CDT. I suspect that the opposite will be true in a few weeks (that calling `Dayjs#tz` on a CST time during CDT will result in a one-hour shift the other way).

And I saw several open issue tickets that are likely related or the result of the same underlying problem:
* #1804
* #1803
* #1801
* #1795
* #1791
* #1690
* #1664
* #1657
* #1635
* #1622
* #1260
* #1001

**Expected behavior**
Applying a timezone should not change the underlying time.

**Information**
- Day.js v1.10.7
- OS: iOS 12.1
- Time zone: GMT-06:00 (Central Standard Time)

Start at the timezone plugin's Dayjs#tz entry point and reproduce the chained calls shown in the report, including dates crossing standard and daylight time. Trace how repeated timezone application calculates offsets; done means applying tz repeatedly preserves the underlying time and the reported examples no longer shift by an hour.

Written by the indexing model from the issue text.