Tune SQLite

This change makes sure the b-tree page size isn't 1024 bytes. It also enables
WAL mode. This means TensorBoard can perform reads at the same time as
TensorFlow is performing writes.

We now also fsync() less often. This shouldn't carry any risk of database
corruption in WAL mode. Since WAL mode uses shared memory, writes become
immediately available to other processes, but they won't become durable until
after the OS decides to flush the FS cache.

This makes the DB writer faster than the file writer, at least in cases where
the DB is tiny. We probably make it go faster still, once we find a way to use
transactions.

Name                      Cold ?s   Average ?s  Flushing ?s       Size B
?i.i                        1,920           69            0            0
Scalar 1.0 FS               1,623          337        4,258       11,348
Scalar 1.0 TB FS            3,137          527        4,213       17,023
Scalar 2.0 FS               3,319          681        3,917       11,348
Scalar 2.0 DB               2,601          578          217      118,784
Tensor 1.0 FS 4             6,397          558        4,276       14,215
Tensor 2.0 FS 4             1,678          613        3,971       24,455
Tensor 2.0 DB 4             3,605          278          313      118,784
Tensor 1.0 FS 128           1,857          289        4,397       47,111
Tensor 2.0 FS 128           3,558          721       10,894       57,351
Tensor 2.0 DB 128           3,508          585          203      118,784
Tensor 1.0 FS 8192          2,677          525        4,400    2,119,816
Tensor 2.0 FS 8192          2,248          822        4,006    2,130,056
Tensor 2.0 DB 8192          4,346          370          449      126,976

PiperOrigin-RevId: 178666363