Out-of-bounds memory allocation request sizes should be treated as just

Out-of-bounds memory allocation request sizes should be treated as just
elog(ERROR) not an Assert trap, since we've downgraded out-of-memory to
elog(ERROR) not a fatal error.  Also, change the hard boundary from 256Mb
to 1Gb, just so that anyone who's actually got that much memory to spare
can play with TOAST objects approaching a gigabyte.