fix(jsonrpc): fix deadlock in get_all_accounts()

`self.accounts.read().await.get_all()` acquires a read lock
and does not release it until the end of `for` loop.
After that, a writer may get into the queue,
e.g. because of the concurrent `add_account` call.
In this case `let context_option = self.accounts.read().await.get_account(id);`
tries to acquire another read lock and deadlocks
because tokio RwLock is write-preferring and will not
give another read lock while there is a writer in the queue.
At the same time, writer never gets a write lock
because the first read lock is not released.

The fix is to get a single read lock
for the whole `get_all_accounts()` call.

This is described in <https://docs.rs/tokio/1.44.1/tokio/sync/struct.RwLock.html#method.read>:
"Note that under the priority policy of RwLock, read locks are not
granted until prior write locks, to prevent starvation. Therefore
deadlock may occur if a read lock is held by the current task, a write
lock attempt is made, and then a subsequent read lock attempt is made by
the current task."

deltachat-rpc-client/tests/test_something.py

@@ -797,3 +797,11 @@ def test_rename_group(acfactory):
         alice_group.set_name(name)
         bob.wait_for_incoming_msg_event()
         assert bob_chat.get_basic_snapshot().name == name
+
+
+def test_get_all_accounts_deadlock(rpc):
+    """Regression test for get_all_accounts deadlock."""
+    for _ in range(100):
+        all_accounts = rpc.get_all_accounts.future()
+        rpc.add_account()
+        all_accounts()