Update counters to LongAdder as appropriate

[myllynen]

Parfait already requires Java 8 which introduced LongAdder. From https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/atomic/LongAdder.html:

This class is usually preferable to AtomicLong when multiple threads update a common sum that is used for purposes such as collecting statistics, not for fine-grained synchronization control. Under low update contention, the two classes have similar characteristics. But under high contention, expected throughput of this class is significantly higher, at the expense of higher space consumption.

Counters implemented with AtomicInteger should be migrated to LongAdded as appropriate.