Release 1.21.0-incubating

This is the third of Apache Pulsar since entering the ASF incubator.

Major changes in this release include:

• [#689] Upgrade to Netty 4.1
• [#846] Publish the shaded pulsar-client as the default dependency
• [#832] [#833] [#849] [#852] Enhancements to Kafka API wrapper to have it work with Kafka's own benchmark tools
• [#836] Fix to C++ partitioned consumer client
• [#822] [#826] Several fixes and improvements related to the namespace bundles
• [#848] Allow consumer to seek to message id from within Pulsar client
• [#903] PIP-8: Scale Pulsar beyond 1M topics
• [#824] Enable secure replication over TLS
• [#923] Upgrade to bk-4.3.1.83-yahoo to expose journalSyncData option
• [#807] Prevent message duplication when active consumer is changed

Complete list of changes can be found at:
https://github.com/apache/incubator-pulsar/milestone/10?closed=1