add support for minute interval

[romanjokl]

This add support for minute interval. timestamp is formatted as time_t / 60 in both file names, client -t parameter etc.

• client list command will list all databases even if there is a hole in the sequence
• client will detect interval format automatically
• client is able to retrieve multiple databases and merge them (sum data)
• fix for cleaning up databases regularly and only write database a second time after merge were included