Create compact block RPCs to allow compact passing of block proposals

[instagibbs]

A functionary layer should be in charge of passing around proposals and finalized blocks, and an important part is taking advantage of the already-built compact block machinery.

All ignoring "PoW checks":

getcompactsketch -> binary format in hex
consumecompactsketch -> fill in sketch as much as possible, return hex if complete, otherwise return list of missing indices
getgetblocktxn -> binary format in hex
consumegetblocktxn -> fills in sketch, returns complete block if complete, error otherwise

Other improvements to improve sync:
prefill transactions up to limit:

• most recent txns

Don't include transactions in block less than X seconds old (already merged)

fixed in #410

[instagibbs]

One wrinkle here is there is no serialization of PartiallyDownloadedBlock, which means clients either have to keep state( 😬 ) or we have to just hand-roll our own for local stateless use.

We could just have a single slot in memory on the node that caches the latest PartiallyDownloadedBlock processed by RPC, so the caller simply cannot check multiple concurrently.

[instagibbs]

WIP here: https://github.com/instagibbs/elements/tree/cmpctsketchrpc

Caching isn't so bad, and involves the least amount of maintenance upstream, so I'm going that route.

[instagibbs]

merged in elements-0.14+