Allow env overrides, query over 50 streams

[aaronfay]

Adds a few improvements:

• Allows MAX_BATCH_ITEMS to be adjusted through the environment variable AWSLOGS_SD_MAX_BATCH_ITEMS, we found that even at 50 we were getting the error Upload too large: 1070727 bytes exceeds limit of 1048576
• Allows the python log level to be set through the environment variable AWSLOGS_SD_LOGLEVEL
• Handles checking more than 50 log streams (by checking for nextToken) before attempting to create a new stream

Thanks for the great project, this is the only one we've found that will forward logs based on systemd units! 🙏