Key-Value Store with Transactions

Design an in-memory key-value store with nested transactions.

Implement the KVStore class (keys and values are strings):

• set(key, value) stores a value.
• get(key) returns the value, or null if the key does not exist.
• delete(key) removes the key (nothing happens if it does not exist).
• begin() opens a new transaction. Transactions nest.
• commit() merges the innermost open transaction into its parent: the changes become part of the enclosing transaction, or of the store itself if it was the outermost. Returns true, or false if no transaction is open.
• rollback() discards the innermost open transaction and all changes made in it. Returns true, or false if no transaction is open.

Reads inside a transaction see its own changes, then those of enclosing transactions, then the store. A change made in an inner transaction is invisible to the outer one until the inner one commits.