Fix/1.6/tx chaining

Here are two commits to fix transaction chaining issues:

• fix #1248 (closed): the protocol was bugged and never allowed chained transactions in a block
• fix #1242 (closed): set the depth limit for chained transactions to 5, from 1st of February 2018

Note: it is possible that forks occur whenever people upgrade to 1.6.15 after 1st February, because chained transactions will be allowed then. So we should either:

• release way before 1st February
• delay the chaining trigger to a later date

cc @nodes/typescript