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/helpserv/src/take.rs

@@ -1,4 +1,4 @@
-use fedserv_api::{Sender, ServiceCtx, Store};
+use echo_api::{Sender, ServiceCtx, Store};
 
 // TAKE <id> (aka ASSIGN): claim a specific ticket.
 pub fn handle(me: &str, from: &Sender, id: Option<&str>, ctx: &mut ServiceCtx, db: &mut dyn Store) {