Optimize weather plugin by reusing manager and other refactors

As documented in the http-client library, calling newManager is an
expensive operation:

```
Creating a new Manager is a relatively expensive operation, you are
advised to share a single Manager between requests instead.
```

But inspite of the haddocks in xmobar claiming that once 'Manager' is
created, it will be used throughout the monitor is not true. Because for
every call of `startWeather` a new manager is being created.

Also I removed the option in WeatherOpts because even if it is false,
it will be ultimately created in `getData` function. Also without
using a manager - the plugin won't really work. So, I don't think
there is any reason for this option to exist.

I have introduced a new dependency http-client-tls to use the shared
global manager so that we reuse the same manager every time. This
simplifies a lot of code. Note that this is not really a new
dependency because http-conduit already depends on it transitively.

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