Prerelease 21.0.0-RC2

This is the second release-candidate for the 21.x.x series.

More detailed release notes are still to be written, use at your own peril.

Relevant changes compared to 20.4.0:

• #143 (+ #163): Dependency updates: We now depend on spring-boot 2.5, Flyway 7, and (optionally) tokens 0.14.0.
  This means that the minimum Java version is now Java 11.
  This is using the new Flyway callback infrastructure. If you are using this, please be careful.

• Feature: (#140, implemented in #141) : Possibility to persist multiple events in bulk (using just one request to the DB).
  This should improve the performance if you create many events of the same type in the same transaction.

We might do incompatible changes between this and the final release of 21.0.0 – only use this if you are prepared to work with this.

Relevant changes compared to 21.0.0-RC1 (which was already in 20.4.0):

• Feature (#112 + #111, implemented in #152): Configurable lock size

  When sending events out, we are first locking them. With this change the number of locked events gets configurable in the nakadi-producer.lock-size property. The lock size defines the maximum amount of events which are loaded into memory and published in one run (in one submission per event type). By default, all events are loaded into memory. In future releases, this property will become mandatory.