Streaming ZIP: avoid hang during central directory write and reduce CPU

[me21]

Problem: Download of ZIP file containing 50000 files stalled while zipfile wrote the central directory; Chrome spinner kept spinning and Python hit 100% CPU.
Root cause: bytes are immutable; _buffer += b caused O(n^2) copying during many small writes at close time. No data was yielded until the end.
Changes
Reworked ZipflyStream to be queue-backed with chunked staging:

• Use a mutable bytearray (_staging) to append writes in-place and cut fixed-size chunks to a bounded queue.Queue (default 64 chunks).
• Convert chunks to immutable bytes before enqueue for thread-safety and to decouple memory from _staging.
• Added flush() to push remaining bytes and close() to flush and enqueue a None sentinel.
• Track total written size without concatenating buffers.

Updated ZipFly.generator() to stream concurrently:

• Start a background writer thread that builds the ZIP (files + central directory) and writes into ZipflyStream.
• Drain the queue in the generator and yield chunks until the sentinel is seen; then join() the writer.
• Set _buffer_size from the stream after completion.

Small import additions: threading, queue.
ZipflyStream.init now takes chunksize (internal use only).