sync-service: Make sqlite durabilty and page cache size configurable

I've dropped the durability of the sqlite shape db from "normal" (where wal appends are fsynced) to "off" where all writes just live in the kernel page cache until automatically flushed and bumped each connection's page cache size to 4MiB to improve performance on EBS volumes.

This is a quick fix - these values should have different defaults (normal durability and a lower page cache) so we need to add config params for this.