Consolidate PG proto parsing-deparsing and backend code.

Now postgres_backend communicates with the client, passing queries to the
provided handler; we have two currently, for wal_acceptor and pageserver.

Now BytesMut is again used for writing data to avoid manual message length
calculation.

ref #118

zenith_utils/Cargo.toml

@@ -5,6 +5,10 @@ authors = ["Eric Seppanen <eric@zenith.tech>"]
 edition = "2018"
 
 [dependencies]
+anyhow = "1.0"
+bytes = "1.0.1"
+byteorder = "1.4.3"
+log = "0.4.14"
 serde = { version = "1.0", features = ["derive"] }
 bincode = "1.3"
 thiserror = "1.0"