sql: start transactions as IMMEDIATE

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.

CHANGELOG.md

@@ -6,6 +6,7 @@
 - use transaction in `Contact::add_or_lookup()` #4059
 
 ### Fixes
+- Start SQL transactions with IMMEDIATE behaviour rather than default DEFERRED one. #4063
 
 ### API-Changes