Find occurrences asynchronously in rename and change-signature

[bbatsov]

The big async conversion. cljr-rename-symbol and cljr-change-function-signature used to locate every occurrence with a blocking call that analyses the whole project - on a cold cache or a large project that freezes Emacs, sometimes for a long time.

They now use a new cljr--find-symbol-occurrences, built on the streaming cljr--find-symbol that cljr-find-usages already uses. It accumulates the occurrences and, when the search finishes, hands the full list to a callback. The command returns immediately; the rename (or the change-signature edit buffer) happens in the callback, so Emacs stays responsive during the analysis.

The callback is deferred out of the nREPL process filter (via a zero-delay timer) so it can safely prompt - the preview confirmation - and pop up buffers. It completes on the middleware's count message or the nREPL done status, surfaces a middleware error instead of letting it escape the filter, and has a generous safety-net timeout so a dead connection can't wedge it forever (10 min - deliberately not a deadline on legitimately slow analysis). Side effect: these commands now honor cljr-middleware-ignored-paths.

Drops the now-unused cljr--find-symbol-sync and cljr--end-of-buffer-p.

Validated live against a real refactor-nrepl connection: rename returned immediately, then previewed + applied across two files on confirm and left them untouched on decline; change-signature returned before its edit buffer existed, the deferred callback opened it, and commit swapped params across two files. Buttercup covers the collector (collection, dedup, error path, done-fallback). This is the highest-value async target from the audit - the remaining sync calls are point-acting or fast, so this is likely where the async work stops.