This field is called `services` in Bitcoin and Zcash, but because we use
that word internally for other purposes, calling it `PeerServices`
disambiguates the meaning to "the services advertised by the peer",
rather than, e.g., a `tower::Service`.
I don't feel super strongly about this change, so I'm happy to drop it,
but it makes the parsing match statements line up nicely and aligns
naming with the naming used in Bitcoin.
This removes the inventory vector structs from `zebra-chain` (as they
are really part of the network protocol) and refactors them into a
single `InventoryHash` type. This corresponds to Bitcoin's "inventory
vector" but with a different, better name (it's not a vector, it's just
a typed hash of some other item).
This provides a significantly cleaner API to consumers, because it
allows using adaptors that convert a TCP stream to a stream of messages,
and potentially allows more efficient message handling.