Delete inactive clients/sessions

[jmattheis]

This MR includes:

• Add createdAt fields to all models
• Add expiresAfterInactivitySeconds to client
• Clean up inactive clients

fixes #943

[codecov]

Codecov Report

❌ Patch coverage is 75.53191% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.90%. Comparing base (ec8ce07) to head (87db368).

Files with missing lines	Patch %	Lines
database/client.go	68.75%	5 Missing and 5 partials ⚠️
router/router.go	0.00%	7 Missing ⚠️
database/database.go	71.42%	2 Missing and 2 partials ⚠️
app.go	0.00%	1 Missing ⚠️
auth/authentication.go	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #959      +/-   ##
==========================================
- Coverage   75.01%   74.90%   -0.12%     
==========================================
  Files          61       62       +1     
  Lines        2886     2953      +67     
==========================================
+ Hits         2165     2212      +47     
- Misses        566      579      +13     
- Partials      155      162       +7     

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[eternal-flame-AD]

Another option for this to work precisely is to memoize an expiresAt field updated along with lastUsed, and predicate all client selections with a condition.

[eternal-flame-AD]

lgtm!