[LibOS] shim_ipc_helper.c: Actually wait for 0.5 seconds on IPC exit
Before exiting, the IPC helper is supposed to wait for 0.5s for all in-flight
IPC messages to reach their destinations (e.g., child processes). Otherwise,
the destination processes may become abandoned zombies. Previously, this was
achieved by `DkThreadDelayExecution(500)`. However, this function uses
microseconds not milliseconds, thus, Graphene actually waited only for 0.005
seconds (which is not enough under heavy system-wide load). This commit fixes
this time-unit bug.