Rename and add license header + javadoc
Rename ProducerInjector to SyncProducerInjector, and DelegatedProducerInjector to ProducerInjector
Add license header to these ProducerInjector.java, SyncProducerInjector.java, AsyncProducerInjector.java and ProducerableInjector.java
Add javadoc header for ProducerInjector class and ProducerableInjector interface