[StreamingHandler] don't hold lock while doing I/O

Change-Id: Ib338b6438bb55a8aba2ca6576fb017b4ab83e08a
Reviewed-on: http://review.couchbase.org/28196
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>