With the introduction of transactions in Contact::add_or_lookup(),
python tests sometimes fail to create contacts with the folowing error:
Cannot create contact: add_or_lookup: database is locked: Error code 5: The database file is locked
`PRAGMA busy_timeout=60000` does not affect
this case as the error is returned before 60 seconds pass.
DEFERRED transactions with write operations need
to be retried from scratch
if they are rolled back
due to a write operation on another connection.
Using IMMEDIATE transactions for writing
is an attempt to fix this problem
without a retry loop.
If we later need DEFERRED transactions,
e.g. for reading a snapshot without locking the database,
we may introduce another function for this.
This also removes BackupProvider::join in favour of implementing
Future directly. I wondered about implementing a FusedFutre to make
this a little safer but it would introduce a dependency on the futures
crate in deltachat-ffi which did not exist yet, so I didn't do that.
We no longer need that in the transfer case, that would give very
weird results. This also means there is nothing imex-specific about
this anymore so move it to blobs.rs
In execute_migration transaction first update the version, and only
then execute the rest of the migration. This ensures that transaction
is immediately recognized as write transaction and there is no need
to promote it from read transaction to write transaction later, e.g.
in the case of "DROP TABLE IF EXISTS" that is a read only operation if
the table does not exist. Promoting a read transaction to write
transaction may result in an error if database is locked.
get_chat_msgs() function is split into new get_chat_msgs() without flags
and get_chat_msgs_ex() which accepts booleans instead of bitflags.
FFI call dc_get_chat_msgs() is still using bitflags for compatibility.
JSON-RPC calls get_message_ids() and get_message_list_items()
accept booleans instead of bitflags now.
