Default chunk size is too large (write timeout)

Hello,
In first, thank you for your plugin!
I'm plaiyng with influxdb to collect my service logs, and I get a problem when putting big logfiles.

Global default of buffer_chunk_limit is 8MB, so influxdb plugin will send a 8MB JSON document at the most.
In this case, influxdb take long time to process the request without responding any bytes.
In result, write request will be aborted by default read_timeout check of net/http.

I suggest two solution;
 * Change default value of buffer_chunk_limit to 512k (or ~1M).
 * Make timeout configurable.

This patch provides both.