Flush the trace stream before displaying sat results

[utaal]

The trace stream can be set to log quantifier instantiations with (set-option :trace true), (set-option :trace-file-name <filename>).

We use the log to profile quantifier instantiations in our verification tool: we use z3's smtlib interface interactively, and we inspect the log after a check-sat returns. Because the trace stream writer is buffered, we witness a partial log of the instantiations if we read it after a sat result is returned but before terminating z3 (we run additional queries on the same z3 process).

This change flushes the trace stream just before reporting a sat result. This seems minimally invasive, but there may be other ways to achieve a similar result that you may prefer, such as adding a new command to flush the trace stream. We could also make this configurable with an additional :trace option.

/cc @parno @Chris-Hawblitzel