[cameron-craig-etas]

Closes #897

This PR fixes a hang during shutdown when there are unhandled events in the event queue. The unhandled events cause the waiting LmControl thread to block, this in turn prevents launch manager from shutting down.

To reproduce this issue, an integration test was written to fire LmControl requests during shutdown. This test reproduces the issue, causing launch_manager to hang. A few fixes were needed to get the test passing consistently:

• On shutdown, complete any GetActiveRunTarget or SetRequestedRunTarget events.
• During the event completion loop, a fix was needed to ensure we are using the available timeout, and not exiting prematurely, before the event queue is cleared.

The test now passes consistently:

bazel test --config=host --compilation_mode=dbg --runs_per_test=100 --local_test_jobs=1 //tests/integration/lm_shutdown_with_control_requests