Fix: Closing kafka Writer during WriteMessages causes a potential hang #1307

[iamgoroot]

Fixes issue #1307 with closing Writer while WriteMessages() is ongoing

https://github.com/segmentio/kafka-go/issues/1307

Cause of the issue:

• Close takes the lock and goes through writers created in batchMessages closing them in a loop
• Close hangs in wait while ongoing messages are processed
• meanwhile in WriteMessages there's only one check for writer being closed. If WriteMessages is past that check while closing - new writers are created that will not be released in Close since it's already in a wait

Solution: WriteMessages should fail with error in case the writer is closed upon acquiring mutex second time and before creating new writers (or map of writers) that won't be closed.