KAFKA-5330: Use per-task converters in Connect

[tbcdns]

Instead of sharing the same converter instance within the worker, use a converter per task.

More details:

• AvroData synchronized cache has a big impact on performance confluentinc/schema-registry#514
• https://issues.apache.org/jira/browse/KAFKA-5330

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk8-scala2.12/4728/
Test FAILed (JDK 8 and Scala 2.12).

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk7-scala2.11/4744/
Test FAILed (JDK 7 and Scala 2.11).

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk7-scala2.11/4766/
Test PASSed (JDK 7 and Scala 2.11).

[asfbot]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk8-scala2.12/4750/
Test PASSed (JDK 8 and Scala 2.12).

[tbcdns]

@ewencp could you review this PR?

[tbcdns]

ping @ewencp :)

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk7-scala2.11/6639/
Test FAILed (JDK 7 and Scala 2.11).

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk8-scala2.12/6624/
Test PASSed (JDK 8 and Scala 2.12).

[tbcdns]

retest this please

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk7-scala2.11/6820/
Test PASSed (JDK 7 and Scala 2.11).

[asfgit]

Refer to this link for build results (access rights to CI server needed):
https://builds.apache.org/job/kafka-pr-jdk8-scala2.12/6806/
Test PASSed (JDK 8 and Scala 2.12).

[ewencp]

LGTM, thanks!