Use heap_inplace_update() to unset pg_database.dathasloginevt

Use heap_inplace_update() to unset pg_database.dathasloginevt

Doing this instead of regular updates serves two purposes. First, that avoids
possible waiting on the row-level lock.  Second, that avoids dealing with
TOAST.

It's known that changes made by heap_inplace_update() may be lost due to
concurrent normal updates.  However, we are OK with that.  The subsequent
connections will still have a chance to set "dathasloginevt" to false.

Reported-by: Alexander Lakhin
Discussion: https://postgr.es/m/e2a0248e-5f32-af0c-9832-a90d303c2c61%40gmail.com