Problem: Thread-Safe Transaction Ledger
Design and implement a thread-safe transaction ledger system that manages account balances with the following requirements:

Core Operations:
* deposit - Add funds to an account
* withdraw - Remove funds
* transfer - Move funds between accounts

Query Operations (must be optimized):
* getTransactions - Return all transactions for an account
* getTransactionsByTimestampRange - Return transactions within timestamp range across all the accounts