*** Reason for rollback ***

Might cause build to hang forever.

b/320630578

*** Original change description ***

Optimize prefetchInputs.

Use a pre-allocated array to hold the intermediate transfers to avoid allocations. Replace some of RxJava code with Futures to avoid RxJava overheads.

This improves the perfromance of prefetchInputs on a large set of inputs from ~400ms to ~16ms.

Fixes #20555.

Closes #20557.

PiperOrigin-RevId: 599135847
Change-Id: Idae6a1c57e634d16091e31e097b16ca97a67e62d