Fix ConcurrentModificationException on commitLater

[migesok]

Invocations of Java consumer's commitAsync should be protected under the consumer semaphore, or you get errors like this when trying to call commitLater concurrently with poll:

java.util.ConcurrentModificationException: KafkaConsumer is not safe for multi-threaded access

[migesok]

I want also to make a minor release when this fix is merged.

[coveralls]

Pull Request Test Coverage Report for Build 14110240777

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 89 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-2.0%) to 92.547%

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Files with Coverage Reduction	New Missed Lines	%
skafka/src/main/scala/com/evolutiongaming/skafka/Offset.scala	1	95.45%
skafka/src/main/scala/com/evolutiongaming/skafka/consumer/Consumer.scala	88	86.43%

Totals
Change from base Build 12864793900:	-2.0%
Covered Lines:	2558
Relevant Lines:	2764

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💛 - Coveralls