Use systemd unit overrides to configure Jemalloc for its superior performance.

Using Jemalloc is generally recommend for long-running Rust services like our server, mostly due to its better heap fragmentation properties. To avoid turning this into additional complexity and build time via the Jemallocator crate during development, this MR applies this as configuration change during development.

While we are at it, this also moves some other overly deployment-specific variables like RAYON_NUM_THREADS (depends on number of vCPU and I/O capacity) as well as UMTHES_BASE_URL (depends on living in the same local network as SNS) into these overrides.