Add a builder API for iterator options

[FGasper]

Is your feature request related to a problem? Please describe

I almost missed that I was creating an iterator with PrefetchValues=false thus:

iterator, err := txn.NewIterator(badger.IteratorOptions{
    Prefix: ...
})

… which is not the intended “default” behavior (going on the values of badger.DefaultIteratorOptions).

Describe the solution you'd like

The Options struct has a nice builder where you do:

badger.DefaultOptions("/path/to/db").WithLogger(myCustomLogger)

The IteratorOptions should have something similar:

badger.DefaultIteratorOptions.WithPrefix([]byte("some.prefix."))

Describe alternatives you've considered

I was lucky & manually caught my inadvertent suboptimal use. Others may not be so lucky.

Additional context

[linear]

BGR-17 Add a builder API for iterator options