Optimize .mark_as_read! and extend to work on relations

[fatkodima]

This PR optimizes .mark_as_read! to be roughly one query for collections and for relations.

Implements

Partly done by #93 - an optimized "one query" solution for scopes is still pending.

from #73 and fixes #80.

This is a an improvement over my previous PR #94, so probably that can be closed.

Implementation is based on INSERT ... ON CONFLICT DO ... like feature on modern databases (aka UPSERT) using gem upsert which uses native support of "upsert" or emulates if db does not provide it and allows to insert many records at once without db constraint violation.

PostgresSQL requires to have a unique constraint (just index is not sufficient) as so upsert gem, as stated on gem's readme so migration to add such was created.

TODO:

• documentation

[coveralls]

Coverage decreased (-0.8%) to 99.16% when pulling a75caf8 on fatkodima:optimize-mark_as_read into e3e9626 on ledermann:master.

[coveralls]

Coverage decreased (-0.8%) to 99.16% when pulling a75caf8 on fatkodima:optimize-mark_as_read into e3e9626 on ledermann:master.

[fatkodima]

@ledermann Rails now supports upsert natively, so no need of external gems.
Are you interested in this PR?

[ledermann]

@fatkodima Jepp, using native upsert from Rails 6.1 would be nice. But the current behavior (using external gem upsert) is still required for Rails < 6.1.

[fatkodima]

We can optimize and use a native upsert for only rails >= 6.1, but ignore for < 6.1. Wdyt?

[fatkodima]

@ledermann As you removed support for rails 6, wdyt on moving this forward?