[ledger] cleanup rust cloud provider

Various cosmetic changes:
 - factor the signalling logic into crate::utils::Signal
 - FutureOrEmpty takes an Option<&mut F>
 - use try_next() instead of next()

TEST=no functional change, already covered by cloud provider validation
tests (fx run-test cloud_provider_memory_diff_tests)

Change-Id: I88923829f9611e2a31746a7a1a4ef60663e1b74b