[cliquet] Timestamp uniqueness by parent_id under heavy load

Original : https://github.com/mozilla-services/cliquet/issues/25

summary: we can improve the timestamp creation for postgres to avoid collisions on very high-load. But the scenario to have that issue is unlikely. (same user hammerting our server)

http://stackoverflow.com/questions/34817760/how-to-generate-unique-timestamps-in-postgresql

No source file or test is named. Start by reading the linked original issue and the PostgreSQL timestamp-uniqueness discussion, then locate the timestamp creation path for records sharing a parent_id. Done means preventing timestamp collisions under concurrent high-load writes without changing normal behavior.

Written by the indexing model from the issue text.