Fix send rate throttling

• [BUGFIX] BBR: mark connections app-limited when pacing is capped and avoid burst token replenishment under a max pacing rate (#605).
• Build: fix variable initialization warning in lsquic_gen_prof (#602).
• Documentation: add notes around internal helpers used in examples (#604).
• Documentation: document -n option in http_server usage (#603).
• CMake: avoid overwriting lsquic config template during install (#601).