Intermittent AutoCompactTest.ReadHalf failures

We're observing intermittent AutoCompactTest.ReadHalf failures. The failures occur with the latest (1.22) and prior (v1.18) releases, and can be observed on multiple architectures (x86-64 + aarch64), kernels and filesystems. E.g.

```
[ 239s] ==== Test AutoCompactTest.ReadAll
[ 239s] iter 1 => 100.040 MB [other 0.000 MB]
[ 239s] iter 2 => 100.040 MB [other 0.000 MB]
[ 239s] iter 3 => 100.040 MB [other 0.000 MB]
[ 239s] iter 4 => 100.040 MB [other 0.000 MB]
[ 239s] iter 5 => 100.040 MB [other 0.000 MB]
[ 239s] iter 6 => 79.524 MB [other 0.000 MB]
[ 239s] iter 7 => 59.008 MB [other 0.000 MB]
[ 239s] iter 8 => 38.492 MB [other 0.000 MB]
[ 239s] iter 9 => 4.103 MB [other 0.000 MB]
[ 239s] ==== Test AutoCompactTest.ReadHalf
[ 239s] iter 1 => 50.017 MB [other 50.023 MB]
[ 239s] iter 2 => 50.020 MB [other 50.021 MB]
[ 239s] iter 3 => 50.020 MB [other 50.021 MB]
[ 239s] iter 4 => 50.020 MB [other 50.021 MB]
[ 239s] iter 5 => 50.020 MB [other 50.021 MB]
[ 239s] iter 6 => 50.020 MB [other 50.021 MB]
[ 239s] iter 7 => 29.504 MB [other 29.505 MB]
[ 239s] iter 8 => 8.988 MB [other 29.505 MB]
[ 239s] iter 9 => 4.103 MB [other 0.000 MB]
[ 239s] /home/abuild/rpmbuild/BUILD/leveldb-1.22/db/autocompact_test.cc:103: failed: 0 >= 9442061
```

```
99 // Verify that the size of the key space not touched by the reads
100 // is pretty much unchanged.
101 const int64_t final_other_size = Size(Key(n), Key(kCount));
102 ASSERT_LE(final_other_size, initial_other_size + 1048576);
103 ASSERT_GE(final_other_size, initial_other_size / 5 - 1048576);
```