Enable setting last timestamp to any value

[dmarsic]

Last timestamp is saved in the inventory database as the current timestamp of the encrypt_all() operation. Timestamps of files in plaindir are compared against this saved value to determine new files to encrypt.

There are valid use cases where we may want to re-encrypt all files, or encrypt files after some timestamp. check.py could provide information about the saved timestamp (in unix timestamp and human-readable format) and also accept the new timestamp:

It seems appropriate that this information is available both when listing all records (check.py with no arguments) and when listing summary (check.py --list).

$ check.py --list
- Registered regular files :     0
- Registered symlinks :          0
- Registered empty directories : 1

Total number of records :        1
Last timestamp:                  1567923006 (2019-09-08T06:10:06Z)

The last timestamp could be changed by specifying unix timestamp as an argument to check.py. Human-writable dates like "1 day ago" could be implemented if we take dependency on Maya, but it's not a hard requirement for this issue.

$ check.py -t|--set-timestamp 1567923006
$ check.py -t|--set-timestamp "1 day ago"   # optional