[#209] Resolve "prevent rate limiting errors"

Closes #209 (closed)

Context

1. don't appear to get rate limited for messages without attachments
2. rate limiting is not exactly 50 messages / min (messages toward the end of a 50 message batch are rate limited, partly because bursty sending produces unpredictable rates of sending)

Changes

• only batch messages with attachments, since messages without attachments do not appear to be rate-limited
• smooth sending traffic for messages with attacments to observed optimal rate of 1 msg / 1.3 sec (batches of size 1 and interval of 1300 millis)