Add option to `influx -import` to override maximum write batch size

`influx -import` currently assumes a maximum write batch size limit of 25 MiB. A command line option should be added to override the this for users that use the `[http] max-body-size` configuration option on their server.

Start at the `influx -import` command entry point and trace where the 25 MiB maximum write batch size is applied. Add a command-line override that accommodates servers using `[http] max-body-size`, then run the existing import tests or a representative import against such a server to verify the configured limit is honored.

Written by the indexing model from the issue text.