Test multiple transactions in one block

[thepiwo]

all transactions should be in the same block

• account A has 100 tokens, spends 100 to B should succeed
• account A has 100 tokens, spends 110 to B should be invalid
• account A has 100 tokens, spends 40 to B, and two times 30 to C should succeed
• account A has 100 tokens, spends 50 to B, and two times 30 to C, last transaction to C should be invalid, others be included
• account A has 100 tokens, spends 100 to B, B spends 100 to C should succeed if (still) undefined is correct

[thepiwo]

do this after #70 so the transaction ordering will be explicitly defined