Make Transformer single-threaded and get rid of `rowSupplierForValueMatcher` ThreadLocal

`Transformer`'s `ThreadLocal rowSupplierForValueMatcher` doesn't need to be an instance field for the same reasons as `IncrementalIndex`'s `ThreadLocal in` field (see #8886). In the case of `Transformer`, it would be actually better to make it a single-threaded class right away and remove a `ThreadLocal` from inside `Transformer`, forcing users of the class to produce copies of `Transformer` per-task (worker, firehose, etc), if they need it.

Start by reading Transformer and compare its ThreadLocal rowSupplierForValueMatcher with IncrementalIndex's ThreadLocal in and issue #8886. Search for Transformer construction and use sites to identify where per-task copies are required. Done means Transformer is single-threaded, the ThreadLocal is removed, and all affected callers provide appropriate copies.

Written by the indexing model from the issue text.