move flushes in display_statistics

[yizhou7]

We encountered issues with Z3 commandline stats display. Specifically in the case where the output is redirected to a file, the results are cutoff upon timeout. For example: z3 test.smt2 -T:3 -st > out_st
Assuming the query timeout after 3 seconds, out_st would only contain the timeout error, but without the rlimit-count, :total-time etc.

Such behavior does not happen when the output is just printed to stdout. The difference might be due to that stdout flushed on newline, but a redirected file doesn't.

This change moves the force flush a bit later, so that the complete output will also get written to a file.