SI-7418 Avoid concurrent use of compiler in REPL startup

Enable the tab completer *after* we're finished binding
$intp and unleashing power mode on the asynchronous
REPL startup thread.

Tested manually:
 - run qbin/scala
 - Paste "".toUp
 - Hit <TAB> like a maniac

Before this patch, the crash was reproducible almost every time.
Afterwards, not the once.

Review by @paulp