Allow the SQLite sync mode to be set independently for WAL.

This change leaves the sync mode at FULL for both WAL and non-WAL
but makes it easy to change it for one but not the other.

To reduce the number of synchronous writes, it might make sense to
change the sync mode for non-WAL to NORMAL instead of FULL which
should be just as safe.

On the other hand, the sync mode for WAL should probably remain FULL
because there may be an impact on transaction durability otherwise.

Initial experiments show that there might not be a significant
performance benefit to using NORMAL, but we may revisit this later.

Change-Id: Ifcd55bedcfefa6600974c2295ca5d4163b408cbf