Add message_batch size checking since SQS can only handle 256KiB

in a batch. Flush queue if message_batch is 10 messages or >= 250KiB

Resolves issue https://github.com/josegonzalez/python-beaver/issues/204, leaves rest of the message handling and organization intact. Leaves ~6KiB of headroom since we are just looking at the raw length of the line and none of the header information, etc. Doesn't require reworking message building in any fashion.