SCR_Have_restart does not always return most recent restart

[mcfadden8]

When running with SCR_FLUSH_ASYNC=1, SCR_Have_restart does not return the most restart written if SCR_Start_output or SCR_Finalize have not been called.

[mcfadden8]

This was reported by a user running the following scenario:

1. Turn on SCF_FLUSH_ASYNC
2. Call SCR_Start_output("restart1")
3. Call SCR_Start_output("restart2")
4. Several minutes later (after the asynchronous cache flushing completed), the application was either killed or crashed.

When the application was run again, it was noted, that SCR_Have_restart would always return "restart1"

We believe that this is happening because the only place where SCR's meta-data is updated to indicate what the latest restart is during at the beginning of SCR_Start_output (which will also wait for the previous async IO to finish if it is not already) and SCR_Finalize (SCR_Finalize was never called because of an application crash).

[mcfadden8]

This is likely related to #532