Add Mutex Lock To Budget Withdrawal

[elraphty]

Context

To avoid race conditions on the budget withdrawal, lock the Business logic to make sure only one call can be made at a time

Design

Use Golang mutex lock to lock the Business logic for each request.

Acceptance Criteria

• I've tested on Chrome
• I've created a test that...
• I have rebased and tested locally before submitting my PR
• I can submit a PR within 2 days of taking the bounty

Here is an example backend test