camel-core - Use StringBuilder instead of StringBuffer

Its mostly in some unit tests, but StringBuilder is better than StringBuffer. Also when biased locking is removed from JVM in the future.

And there is this on left

camel-base/src/main/java/org/apache/camel/processor/aggregate/StringAggregationStrategy.java: StringBuffer result; // Aggregate in StringBuffer instead of StringBuilder, to make it thread safe

git grep StringBuffer