fix(datepicker): handle multiple input changes once

Previously internally calling

```ts
service.minDate = ...;
service.maxDate = ...;
```
resulted in two observable emissions, one per input change.

Now the API is changed to:

```ts
service.set({minDate: ..., maxDate: ...})
```

which results in a single emission for any number of inputs changed at the same time.

Fixes ng-bootstrap#3545