…pache#7305)

A partition log in initialized in following steps:

1. Fetch log config from ZK
2. Call LogManager.getOrCreateLog which creates the Log object, then
3. Registers the Log object

Step #3 enables Configuration update thread to deliver configuration
updates to the log. But if any update arrives between step #1 and #3
then that update is missed. It breaks following use case:

1. Create a topic with default configuration, and immediately after that
2. Update the configuration of topic

There is a race condition here and in random cases update made in
second step will get dropped.

This change fixes it by tracking updates arriving between step #1 and #3
Once a Partition is done initializing log, it checks if it has missed any
update. If yes, then the configuration is read from ZK again.

Added unit tests to make sure a dirty configuration is refreshed. Tested
on local cluster to make sure that topic configuration and updates are
handled correctly.

Reviewers: Jason Gustafson <jason@confluent.io>