Revisit indexOfSlice performance improvements

https://github.com/apache/pekko/pull/2306#pullrequestreview-3352698604

PR2306 already improves performance quite a bit.

There is a suggestion that we could eke out some more perf gains by using the https://en.wikipedia.org/wiki/Two-way_string-matching_algorithm - see the link above - basically trying to port Netty's code

Start by reviewing PR2306 and its linked review, then locate the current indexOfSlice implementation in the Pekko source. Compare it with Netty's implementation of the two-way string-matching algorithm and establish a benchmark for the proposed improvement; done means a validated performance gain without regressions.

Written by the indexing model from the issue text.