_bt_check_unique() failed to invalidate binary search bounds in the
event of a live conflict following commit e5adcb78.  This resulted in
problems after waiting for the conflicting xact to commit or abort.  The
subsequent call to _bt_check_unique() would restore the initial binary
search bounds, rather than starting a new search.  Fix by explicitly
invalidating bounds when it becomes clear that there is a live conflict
that insertion will have to wait to resolve.

Ashutosh Sharma, with a few additional tweaks by me.

Author: Ashutosh Sharma
Reported-By: Ashutosh Sharma
Diagnosed-By: Ashutosh Sharma
Discussion: https://postgr.es/m/CAE9k0PnQp-qr-UYKMSCzdC2FBzdE4wKP41hZrZvvP26dKLonLg@mail.gmail.com