* wip
* exhaustive match
* write with fixed v1 strategy when writing for hashing
* local protocol version is 2
* cleanup "size" tests that exercise v1 vs v2 vs default protocol versions
* add proto version to Connected! log msg
* cleanup docs
* negotiate protocol version min(local, peer) when doing hand/shake
* introduce HeaderVersion (was u16) for type safety
cleanup pow ser/deser (version unused)
* fixup tests for HeaderVersion
* validate header version during header deserialization
* expose leaf pos iterator
use it for various things in txhashset when iterating over outputs
* fix
* cleanup
* rebuild output_pos index (and clear it out first) when compacting the chain
* fixup tests
* refactor to match on (output, proof) tuple
* add comments to compact() to explain what is going on.
* get rid of some boxing around the leaf_set iterator
* cleanup
