This reverts commits 3da73d68 and de87a084.

This code has some tricky corner cases that I'm not sure are correct and
not properly tested anyway, so I'm reverting the whole thing for next
week's releases (reintroducing the deadlock bug that we set to fix).
I'll try again afterwards.

Discussion: https://postgr.es/m/E1hbXKQ-0003g1-0C@gemulon.postgresql.org