fix(lib): let an unanswered upload offer expire

An upload offer counted against the slot cap until the peer accepted
it, and a peer that never answered, having gone away or hung, held that
slot for the life of the process; with two slots by default, two such
peers shut uploads down until a restart. An offer nobody answers within
thirty seconds is now dropped, recorded as a failed upload so the
operator sees why, and the slot goes to the next in line.

soulseek-rs-lib/src/client/mod.rs

@@ -107,6 +107,8 @@ struct UploadJob {
     real_path: std::path::PathBuf,
     virtual_path: String,
     size: u64,
+    /// When the offer went out; `None` once the peer has answered it.
+    offered: Option<Instant>,
 }
 
 /// Live bookkeeping for an upload being served (or recently finished).