[FIX] Disallow multiple subscriptions to account transactions

It fixes the issue with notifications appearing twice, and makes the client faster.
Merge pull request #1607 from vhpoet/cur

[FIX] History: Disable currency filter in code
[TASK] Jsonrewriter: Check DeliveredAmount when creating transaction.amountSent
[FIX] History: Show amountSent instead of amount where applicable