Wait for sufficient amount of transaction confirmations

[Eknir]

Currently, we are returning from submitting / cashing a cheque right after we get the first confirmation. Due to the nature of the blockchain we should wait 25 blocks to be reasonably certain that the transaction will always be part of the longest chain. If we don't do this, we risk that our transaction becomes orphaned and the swarm peer will not be aware of this.

[ralph-pichler]

As discussed on a recent call this number should be configurable.