[Identity] Use references of DatabaseClient

Summary:
DatabaseClient is essentially an instance factory, we
don't need to be taking ownership of it on every function call;
thus forcing a .clone() on the caller.

Test Plan:
This will be tested more thoroughly in a later diff, however,
the changes should just reduce the number of deep copies we do
of the "instance factory".

cd services/identity
cargo build

Reviewers: varun, max, bartek

Reviewed By: varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6926