Transmission 4.00

What's new in this version:

- Resource Efficiency - Use less memory and fewer CPU cycles
- Better Community - Pull requests welcomed and used
- Code Modernization - Migrated from C90 to modern C++
- New Features - What would a major release be without them?

Resource Efficiency:
- The code has been extensively profiled and improved to fix inefficient code and memory use. For example, a stress test of starting transmission-daemon with 25,000 torrents is almost entirely IO-bound, using 50% fewer CPU cycles and 70% fewer memory allocations than Transmission 3.00.
- The remote control GUIs (transmission-qt and transmission-web) now use the RPC API "table" mode, resulting in smaller payloads / less bandwidth use.
- RPC payloads are now compressed using libdeflate, a "heavily optimized [library that is] significantly faster than the zlib library."

Community:
- The project is much more responsive to bug reports and code submissions than it has been in the past
- There is a new group of volunteer contributors who are working on Transmission
- Transmission 4.0.0 includes over 350 new community commits (see the Thank You section below and in the previous 4.0.0 betas) since 3.00 and welcomes new contributors
- Documentation has been moved into the transmission/transmission so that contributors can submit PRs to improve it

Code Modernization:
- The entire codebase has been migrated from C to C++. In the process, we've removed thousands of lines of custom code and used standard C++ tools instead. The core's code has shrunk by 18%. The core codebase has been extensively refactored to be more testable and maintainable.
- The GTK client has been ported to gtkmm
- The Web client has been rewritten in modern JavaScript and no longer uses jQuery. The entire gzipped bundle is now 68K.
- The unit tests have been expanded and ported to Google Test. Clang sanitizer builds are run during CI.
- The core library is now fuzz tested
- Transmission now uses Sonarcloud, Coverity, LGTM, and clang-tidy static analysis on new code. Several hundred code warnings have been fixed compared to Transmission 3.00.

New Features:
- Support for using BitTorrent v2 torrents and hybrid torrents. (Support for creating v2 and hybrid torrents is slated for an upcoming release.)
- Users can now set "default" trackers that can be used to announce all public torrents.
- Newly-added seeds can start immediately and verify pieces on demand, instead of needing a full verify before seeding can begin
- Added an option to omit potentially-identifying information (e.g. User-Agent and date created) when creating new torrents
- The Web client has been rewritten and now supports mobile use.
- When creating new torrents, users can now specify the piece size
- IPv6 blocklists are now supported
- Beginning with 4.0.0-beta.1, Transmission releases now use semver versioning.
- Dozens of other new features -- too many to list here! We've been working on this for a year!