pyroblack 2.7.6

PyroBlack v2.7.6 — parallel media sessions & network optimizations

- Introduce multi-session pool for media transfers (download/upload)
  with configurable max_download_workers (default 4), enabling
  parallel chunk fetching across multiple TCP sessions per DC
- Refactor get_file() with pipelined concurrent chunk dispatch and
  proper in-flight task tracking and cancellation
- Rewrite save_file() with multi-session round-robin workers,
  5-retry upload logic, dead-session eviction, and larger read buffer
- Pre-warm media session pool on client initialization
- Tune TCP sockets: TCP_NODELAY, SO_KEEPALIVE, 4 MiB send/recv buffers
- Scale CryptoWorker threads adaptively (cpu_count, capped at 4)
- Consolidate media session creation into shared _make_media_session /
  _get_media_session_pool on Client, replacing inline duplication
- Clean up dead imports, commented code, and redundant docstrings
  across compiler, crypto, file_id, session, and utilities modules

Inspired-by: rejected PR #4 by @Cobbn

Signed-off-by: irisXDR