Rename the project to Echo (was fedserv)

Rebrand the daemon and workspace: the binary is now `echo`, every crate
is `echo-*` (`echo-api` and the module crates), Rust imports use
`echo_*`, the gRPC proto is `proto/echo.proto` with package `echo.v1`,
and the log filter, gossip peer-name default, and on-disk store default
(`echo.db.jsonl`) follow. Docs updated throughout. No behaviour change;
crate directories and the config schema are untouched.

modules/hostserv/src/lib.rs

@@ -3,7 +3,7 @@
 //! with SET/DEL and review with LIST. `lib.rs` holds the dispatcher; each
 //! command lives in its own file.
 
-use fedserv_api::{NetView, Priv, Sender, Service, ServiceCtx, Store};
+use echo_api::{NetView, Priv, Sender, Service, ServiceCtx, Store};
 
 #[path = "on.rs"]
 mod on;