* if the user is root, this is typically in docker and the
database is usually under postgres user
* if the user is something else, typically a psql user exists
for the user so we attempt to use that.
* we set the user once in run_all_tests instead of running the same
logic again in test_runner

Now when usearch external retriever is initialized, an opeque
is passed in which then is passed back in retrievers.
This way, we no longer need to have global variables to manage memory
on which external index nodes live.

Get rid of num_blocks in index header

Cleanup and format