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-jsonrpc/src/api.rs

@@ -227,8 +227,9 @@ impl CommandApi {
     /// Get a list of all configured accounts.
     async fn get_all_accounts(&self) -> Result<Vec<Account>> {
         let mut accounts = Vec::new();
-        for id in self.accounts.read().await.get_all() {
-            let context_option = self.accounts.read().await.get_account(id);
+        let accounts_lock = self.accounts.read().await;
+        for id in accounts_lock.get_all() {
+            let context_option = accounts_lock.get_account(id);
             if let Some(ctx) = context_option {
                 accounts.push(Account::from_context(&ctx, id).await?)
             }