Make the accounts thread safe

[TheoKah]

When multiple give/take/set commands arrive at the same time the final amount was not correct.

You can try that by copy/paste a few /econ add X A commands in a file and copy/paste them all at the same time in console.

[liach]

this code doesn't look.very easy to maintain... i hope you can use braces around the lock blocks and use finally to unlock.

[TheoKah]

Do you prefer this one @liach ?