A concurrency problem in `SQLiteWallet` because of across execution

[nan01ab]

Describe the bug In SQLiteWallet, it uses two lock to avoid concurrency problems:

• lock (accounts) to protect the accounts structure;
• lock (db_lock) to protect the WalletDataContext;

Because there are two locks, and there not locked at the same time. This method may lead to some problems in some cases, for example:

1. Cross-execution of the CreateAccount and the DeleteAccount: These two methods use same lock step:

lock (accounts) { 
  // step 1;
}

lock (db_lock) {
 // step 2;
}

It may lead to unconsistency state if a thread executed AddAccount step 1 and switched out, then another thread executed DeleteAccount step 1 and step 2, then thread-1 executes AddAccount step 2.

2. Cross-execution of diffirent CreateAccounts: Similar execution process.