R1:b3b1cc64bdd0

When applying transactions, acquire a read lock sooner

Summary:
Depends on D20461. Ref T13276. Ref T13054.

Currently, we acquire the transaction read lock after populating "old values" in transactions and filtering transactions with no effect.

This isn't early enough to prevent all weird chaotic races: if two processes try to apply a "close revision" transaction at the same time, this can happen:

```
PROCESS A PROCESS B
Old Value = Open Old Value = Open
Transaction OK: Yes…