Gmoshkin/release preparation

Summary

• release: 4.0.0

• fix: async-h1-client example didn't compile with feature tls

• fix: used to panic in debug mode sometimes

• fix: used to drop join handles in net_box sometimes

• fix: non-joinable fiber func used to capture dangling references

• fix: set_error! macro used to not compile in a value context

• fix: broken test on tarantool-2.10 + better error message

• fix: disallow fiber names with nul-byte via lua api

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• Version is bumped in the appropriate Cargo.toml files