bench: explicitly make all current benchmarks "high" priority

no-functional changes. Only have set the priority level explicitly
on every BENCHMARK macro call.

src/bench/mempool_eviction.cpp

@@ -132,4 +132,4 @@ static void MempoolEviction(benchmark::Bench& bench)
     });
 }
 
-BENCHMARK(MempoolEviction);
+BENCHMARK(MempoolEviction, benchmark::PriorityLevel::HIGH);