Log archiving T3 module

[vbrinnel]

Logs produced by AMPEL will be need to be archived at some point.
Regular AMPEL logs will stay a long time in the DB, logs produced by alert rejections will have a much shorter time to live (weeks).

Roughly, it will:

• convert ObjectId into date string
• bring optional dic key/values ("channel(s)", "tranId", etc...) into a fixed format
• add the log entries to a file
• compress the file (lzma)
• delete the log entries (a fast collection delete will be used for rejection log entries)