Fun quirk of sqlite is that if you BEGIN DEFERRED and then do a read
statement and then another process concurrently does a write and then
the first process does a write statement on the same transaction, it
will immediately return SQLITE_BUSY because it's impossible to do a
write against a past version of the database. To fix this we need to use
BEGIN IMMEDIATE to take a write lock upfront on any transaction that
will need to write.
