The default cache size for a SQLite connection is 2000KiB, but our
database pool has several reader connections plus a writer connection.
They can't share a cache because that changes SQLite's locking model,
so the next best thing is to limit their caches individually when
we're in a memory-constrained environment. To avoid the caches getting
*too* small, this also removes one of the available readers outside
the main app.
Fixes an issue where users were no longer receiving messages after
restoration. During restoration, the restoring device copies the
transferred database files to a special "hotswap" directory since we
don't want to overwrite our currently in-use database. Once finished, it
re-opens the database pointing to our hotswap directory. It would then
move the hotswap database to its primary location on the next app
launch.
The problem here is our extensions don't know how to read the hotswap
directory. Even if we added that capability, it's going to be tricky to
coordinate which database they read and when as the main app shuffles
files around.
This fix adjusts our restoration flow. Instead of having a special
"primary" and "hotswap" directory that we need to fix up on the next
launch, we instead record a UserDefaults entry that points to our
current database directory.
Once transfer has completed, the main app only needs to update the
current database directory in UserDefaults. No post-launch swapping
needs to occur.
Extensions will listen for updates to this database location by
registering KVO on NSUserDefaults.
This change also adjusts the restoration flow to break it up into
discrete stages. The motivation here is we need to make sure that a
partial restoration doesn't put the extensions in an inconsistent state.
