[cleanup][broker] Direct get lowest PositionImpl from TreeMap

[lifepuzzlefun]

Search before asking

• I searched in the issues and found nothing similar.

Motivation

some interface already return TreeSet but the signature is Set
so the caller will use set.stream().findFirst()
which is unnessary.

Solution

No response

Alternatives

No response

Anything else?

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!