bug: fix _expiry() function to use seconds

[jrconlin]

Also enforce expiry on router records

Closes #1073

[jrconlin]

Please refer to #1073 (comment) for why filterExpression is not being used here.

FWIW, the core problem is that if the key identified by ExclusiveStartKey can't be found (because, say the record expired and was dropped by the database), the query fails with a Validation error and the backfill fails.

[codecov-io]

Codecov Report

Merging #1148 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1148   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files          60      60           
  Lines        9923    9938   +15     
======================================
+ Hits         9923    9938   +15

Impacted Files	Coverage Δ
autopush/utils.py	100% <ø> (ø)	⬆️
autopush/db.py	100% <100%> (ø)	⬆️
autopush/tests/test_integration.py	100% <100%> (ø)	⬆️
autopush/tests/test_db.py	100% <100%> (ø)	⬆️

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[pjenvey]

so snafu. nice catch