Add platform condvar abstraction and cancellable busy handler

- Add timed_wait_t abstraction over pthread_cond_timedwait (POSIX) and
  SleepConditionVariableCS (Windows) - ~30 lines covering 100% of BEAM platforms
- Extend connection_t with cancel_tw condvar, cancelled flag, busy_timeout_ms,
  and env+pid stashing for process-alive checks
- Replace sqliteDefaultBusyCallback with exqlite_busy_handler that waits on
  condvar (instantly cancellable) and checks enif_is_process_alive
- Add exqlite_progress_handler to interrupt long-running VDBE execution
- Add exqlite_set_busy_timeout NIF (stores timeout without destroying handler,
  unlike PRAGMA busy_timeout which silently replaces custom handlers)
- Add exqlite_cancel NIF (sets flag, signals condvar, calls sqlite3_interrupt)
- Make cancel and set_busy_timeout non-dirty NIFs (must run on normal scheduler
  to avoid queueing behind stuck dirty NIFs)
- Update destructor to signal cancel before close
- Install handlers in exqlite_open, stash env+pid at 5 busy-triggerable call sites

Fixes the busy handler plane of #192 - disconnect can now wake sleeping handlers
instantly instead of waiting for busy_timeout to expire.

Author: mjc