S3A statistic collection underrecords bytes written in helper threads

Applications collecting per-thread statistics from S3A get underreporting of bytes written, as all byte written in the worker call update those in a different thread.

Proposed:

• the bytes upload statistics are uploaded in the primary thread as a block is queued for write, not after in the completion phase in the other thread
• final WriteOperationsHelper.writeSuccessful() takes the final statistics for its own entertainment

Really I want context-specific storage statistics.