refactor(processor): Limit goroutines in sync mode & simplify (#334)

* feat(processor): Add worker support for sync workloads

* Cancellable

* test: Add unit tests for batch item processor

* feat: Static workers

* more debugging

* Refactor workers

* Add comments

* Close

* test: Add queue size and dropped events tests

* refactor: Improve error handling in enqueueOrDrop function

* refactor: Remove duplicate fields and unused variables in testBatchExporter