Core: Use toOptional and onlyElement methods in MoreCollectors

[ebyhr]

The existing findFirst silently hides multiple matching elements.

This PR replaces it with Guava MoreCollectors:

onlyElement method:

A collector that takes a stream containing exactly one element and returns that element. The returned collector throws an IllegalArgumentException if the stream consists of two or more elements, and a NoSuchElementException if the stream is empty.

toOptional method:

A collector that converts a stream of zero or one elements to an Optional.
Throws: IllegalArgumentException - if the stream consists of two or more elements.
Throws: NullPointerException - if any element in the stream is null.

I only updated a few places to demonstrate how to use them.

[singhpk234]

Are there use case for this for non test scenarios ?
On an orthogonal note it seems like the tests are just fine without this

[ebyhr]

@singhpk234 Yes, the same pattern persists even in non-test code. I've made additional modifications, and there are actually more places where this pattern occurs.