Persistent process for erpc batch operations.

Instead of spawning a new process per erpc.call, clients start a long-lived worker on the remote node and send batches via GenServer.call/2 over Erlang distribution. This eliminates per-call process spawn overhead and distribution codec setup.

No Ranch, no TCP, no RESP parsing -- just distribution to Router.

Usage (from remote node)

# Start a worker on the FerricStore node (once per client process)
{:ok, pid} = :erpc.call(node, FerricStore.BatchWorker, :start, [])

# Send batches -- reuses the same process
values = GenServer.call(pid, {:batch_get, keys})
results = GenServer.call(pid, {:batch_set, kv_pairs})
results = GenServer.call(pid, {:batch_mixed, pipe_commands})

# When done
GenServer.stop(pid)