Tester: fix concurrent map access

• add mutex to codex-map in tester to prevent concurrent map access

Log-Prefix, Emitsync with headers & Bugfix

• Add EmitSyncWithHeaders to Emitter (by @adw1n)
• Prefix sarama logger with [Sarama] (by @adw1n)
• Bugfix in example (by @FredrikBakken)

Fix Monitoring Issue

Fixing the processor and view state in the monitoring web-ui.

Fix issue with sarama 1.27.0

Add Commit() to MockConsumerGroupSession and cgSession to be compatible with ConsumerGroupSession interface.

Sarama consumer group

With this release we integrate sarama consumer group into goka and remove the sarama-cluster dependency. These changes enable users to use goka with recent sarama versions.

bugfix stats tracking

removed race condition in stats tracking
fixed panic in View.Get for uninitialzed state
call Open on storage after init

Bugfixes Shutdown/Rebalance

v0.9.0-beta2

fix rebalancing by adding a copartitioning rebalance strategy

Consumer Group Refactoring

Major changes:

• Removed sarama-cluster, replaced with sarama's ConsumerGroup
• remove package kafka, move remaining files to root
• introduce go mod
• removed support for librdkafka

See #239 for details

Headers + Fixes

Merge pull request #208 from WideLee/master

repeating messages from input stream after recover

rebalance callback

Merge pull request #196 from gmather/feature/195_rebalance_callback

feature 195: Add a rebalance callback to the processor.