BCH Nov2018 HF: min tx size is 100 bytes

[svyatonik]

on top of #525
these commits only: ad4f51a + 18a2766
part of #519

This PR introduces new consensus rule - all post-HF transactions must be > than 100 bytes.

P.S. we should have some kind of trigger which is fired when HF block appears + clears the memory pool. Otherwise memory pool could contain invalid transactions. (submitted an issue #527)

[dagurval]

I'm not familiar enough with the code base to verify this, so I'll ask

• Do you also verify that the coinbase is >= 100 bytes?
• Do you at some point create a coinbase transaction in the codebase? In that case, you'll need to make sure it's the right size

[dagurval]

Nit: Can you void a call to serialized_size by checking if self.min_transaction_size == 0 first?

[svyatonik]

@dagurval Appreciate your review! Fixed a nit. Regarding coinbase: (1) this check affects coinbase (2) we only support getblocktemplate RPC which returns coinbasevalue, but not the coinbasetxn. So coinbase is composed externally.