WIP: Fix kassenbuch get_buchungen for microseconds

[mgmax]

Using Datetime() in the SQLite query strips away the microseconds. SQLites cannot do datetime formatting for microseconds, even with strftime().

First workaround: Discards all microseconds when filtering for a specific time range.

Problem: This may create a race-condition if a booking is made at xxx.100 seconds and then kassenbuch show is called at xxx.200 sec, because the booking was made after xxx.000 sec and the snapshot_time mechanism filters out the booking from the "future".

[codecov-io]

Current coverage is 28.33% (diff: 95.83%)

Merging #143 into development will increase coverage by 0.14%

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