pgsql: Revert "Avoid spurious deadlocks when upgrading a tuple lock"

Revert "Avoid spurious deadlocks when upgrading a tuple lock"

This reverts commits 3da73d6839dc and de87a084c0a5.

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

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/03964e58efb8cbb8ecf3f4cdd27c8c757c30e89e

Modified Files
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src/backend/access/heap/README.tuplock | 10 --
src/backend/access/heap/heapam.c | 83 +++---------
.../expected/tuplelock-upgrade-no-deadlock.out | 150 ---------------------
src/test/isolation/isolation_schedule | 1 -
.../specs/tuplelock-upgrade-no-deadlock.spec | 57 --------
5 files changed, 21 insertions(+), 280 deletions(-)