Refactor template/producer to not use default topic name

[onobc]

This proposal does the following (reason in sub-bullets):

• Introduces a PulsarOperations interface
  • matches spring-kafka and defines contract
  • provides default delegation methods to avoid cookie cutter code in impl
• Removes producer caching from PulsarTemplate (will add this back in on follow commit but probably in producer factory)
  • Current caching is danger of OOME (no cleaning of cache)
• Producer factory no longer holds onto a single producer instance
  • we hand out N of these, if anything we should hold reference to all and close etc..
• Removes defaultClientName from PulsarTemplate
  • not thread-safe (topic set on producer config in producer factory)
  • misleading API and it mutates the config props (I never liked this about KafkaTemplate)

Fun facts

• PulsarClient holds onto all Producers it spits out
• PulsarClient closes all producers on its close
• PulsarClient removes producer from list when producer is closed directly
• Pulsar producers are bound to a single topic at creation time