sql: replace .get_conn() interface with .call()

.call() interface is safer because it ensures
that blocking operations on SQL connection
are called within tokio::task::block_in_place().

Previously some code called blocking operations
in async context, e.g. add_parts() in receive_imf module.

The underlying implementation of .call()
can later be replaced with an implementation
that does not require block_in_place(),
e.g. a worker pool,
without changing the code using the .call() interface.

CHANGELOG.md

@@ -7,6 +7,7 @@
 - Organize the connection pool as a stack rather than a queue to ensure that
   connection page cache is reused more often. #4065
 - Use transaction in `update_blocked_mailinglist_contacts`. #4058
+- Remove `Sql.get_conn()` interface in favor of `.call()` and `.transaction()`. #4055
 
 ### Fixes
 - Start SQL transactions with IMMEDIATE behaviour rather than default DEFERRED one. #4063