verify coinbase maturity via output_mmr_size

[antiochp]

We do not need Merkle proofs to spend coinbase outputs.
We only need the output_mmr_size from the block header.

• verify coinbase maturity via output pos vs. output_mmr_size committed to in block header
• wallet does not need to be concerned with Merkle proofs or block hashes

TODO -

• any more cleanup?
• test with real running nodes

Resolves #1191.
Resolves #1185.

[yeastplume]

Very happy with these simplifications if it all works.. refresh_missing_block_hashes in particular is a bit source of slow-down when trying to use the wallet.

[antiochp]

Going to merge this.
Tests are all passing on travis.
Testing locally with a couple of nodes indicates this is all still working as expected without needing Merkle proofs on the inputs.