Implement dynafed block header support for elements >= 0.18.1

[cdecker]

This implements support for the upcoming elementsd dynafed block header
format, which is slightly different from either the bitcoin block header
format as well as the elementsd <= 0.17.0 block header format. A couple of
fields were added, 3 different serialization formats for the information
(null, compact and full) and some of those fields are not hashed into
the block hash, since they'd be self-referential (like the pre-0.18.0 solution
field).

In order to implement this I chose to also simplify the block hash generation,
to happen in parallel with the deserialization. This has a couple of
side-effects:

• The block hash is computed only once, instead of multiple times when we
  call bitcoin_block_blkid. For compatibility I left that function, but it
  now just copies over the cached hash instead of re-serializing it.
• We do not need any serialization code for the block header, since we are
  just consumers of blocks, and we don't have to generate new ones from its
  constituent parts.
• We don't need to carry block header fields around that are otherwise of no
  use to us. It always felt weird having to remember constituent parts only
  to be able to re-serialize the header. Now we can just feed it into the
  hashing algorithm and skip the actual fields. This also ends up saving us a
  couple of allocations.

There are a couple of drive-by fixes, such as weening off of elementsd's
deprecated generate RPC in favor of generatetoaddress, but they should be
minor.

Fixes #3362

[cdecker]

As a minor caveat, there are intermediate versions (v0.18.0, v0.18.1.1,
v0.18.1.2) that predate the introduction of the elided_root in the block
header params when serialized in the compact format. However, there is no
programmatic way to identify whether the block header will contain the field
or not, since the version field was not updated. In this PR I chose to
implement the parsing that includes the elided_root, therefore running any
of the versions above results in an incompatibility and will crash
lightningd. You've been warned ⚠️

More details on the rationale for the change can be found here: ElementsProject/elements#773

[rustyrussell]

Ack fa8248a