Use uuid in tmp db names in unit tests

[eiri]

Overview

This changes naming of temporary eunit databases and files from using unique_monotonic_integer to couch_uuids:random to remove performance penalty and normalize naming across erlang releases.

Also getting rid of unnecessary inter-types convertion.

Testing recommendations

make eunit should come clean. Files in $PWD/tmp should be named like tmp/data/eunit-test-db-a6a4cd31f2addf2791440b3e8b54f860.1515084353.couch or tmp/tmp_data/nonode@nohost-018448e4ebb1f85812fd96d56939a620

Checklist

• Code is written and works correctly;
• Changes are covered by tests;
• Documentation reflects the changes;