binderThroughputTest: gather servers right away

Start gathering servers upon worker creation. Don't wait until signaled
by test. This should allow all clients to start transactions at a better
coordinated time.

Change-Id: I4568eb55fc9ce18839c1e5592a722a0284987745
Test: Ran a benchmark on cuttlefish, and it did not get stuck
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Co-developed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>