Consider transaction finality depending on its size

[davidyuk]

I think there is a 100 in the code base that represents the longest chain that is ever going to be accepted, so in that sense “finality” is 100 generations. In practice, I’d wait 2 microblocks for small TXs, 2 generations for medium TXs and 4-5 for large TXs.

Suggested by Hans Svensson