Prevent concurrent withdraw of one account

[maryarm]

Describe the bug
Right now it's allowed If two or more transfer from one account happens at the same time that leads to wrong rule checking or even empty balance.

To Reproduce
Steps to reproduce the behavior:

1. Account A has 10BTC
2. Account A is going to transfer 10BTC to B
3. Same time Account A is going to transfer 10BTC to C
4. Account A's balance is -10BTC

Expected behavior
The second transfer fails