As a work-around for limitations in the size of strings allowed by
Redis, we want to be able to split large inputs into several reports.
This commit adds basic support code for this together with corresponding
unit tests.

Part of issue certtools#547

The file collector bot now has two new parameters, chunk_size and
chunk_replicate_header, controlling whether the input is to be split
into chunks and how big those chunks should be and what to do with (CSV)
header lines.

Part of issue certtools#547

The mail URL collector bot now has two new parameters, chunk_size and
chunk_replicate_header, controlling whether the input is to be split
into chunks and how big those chunks should be and what to do with (CSV)
header lines.

Part of issue certtools#547

…-csv-reports

…v-split-csv-reports

Manually resolved two conflicts. Thei fields feed.name and feed.code are now
set by the CollectorBots __add_report_fields method.

Conflicts:
	intelmq/bots/collectors/file/collector_file.py
	intelmq/bots/collectors/mail/collector_mail_url.py

Fixed merge conflicts in:

Conflicts:
	intelmq/bots/collectors/file/collector_file.py
	intelmq/bots/collectors/mail/collector_mail_url.py