Use cancel in disconnect and NIF for set_busy_timeout

- Replace Sqlite3.interrupt(db) with Sqlite3.cancel(db) in disconnect/2
  (cancel is a superset that also wakes the busy handler condvar)
- Replace PRAGMA busy_timeout with Sqlite3.set_busy_timeout NIF call
  (PRAGMA internally calls sqlite3_busy_timeout which destroys custom handlers)

Together these changes complete the fix for #192 - disconnect now properly
cancels all three blocking planes (VDBE execution, busy handler sleep, and
mutex contention as a side effect of the first two).

Author: mjc

c_src/sqlite3_nif.c

@@ -1989,7 +1989,10 @@ exqlite_set_busy_timeout(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
         return enif_make_badarg(env);
     }
 
+    // Protect write to busy_timeout_ms since busy handler reads it
+    tw_lock(&conn->cancel_tw);
     conn->busy_timeout_ms = timeout_ms;
+    tw_unlock(&conn->cancel_tw);
 
     return am_ok;
 }

lib/exqlite/connection.ex

@@ -212,10 +212,11 @@ defmodule Exqlite.Connection do
       apply(state.before_disconnect, [err, state])
     end
 
-    # Interrupt any in-flight query so close() doesn't block on conn->mutex.
-    # Without this, a long-running dirty NIF holds the mutex and close() deadlocks.
+    # Cancel any in-flight query: wake the busy handler condvar AND interrupt
+    # VDBE execution so close() doesn't block on conn->mutex.
+    # This is a superset of the old Sqlite3.interrupt(db) call.
     # See: https://github.com/elixir-sqlite/exqlite/issues/192
-    Sqlite3.interrupt(db)
+    Sqlite3.cancel(db)
 
     case Sqlite3.close(db) do
       :ok -> :ok
@@ -513,7 +514,9 @@ defmodule Exqlite.Connection do
   end
 
   defp set_busy_timeout(db, options) do
-    set_pragma(db, "busy_timeout", Pragma.busy_timeout(options))
+    # Use our NIF instead of PRAGMA busy_timeout, because PRAGMA internally
+    # calls sqlite3_busy_timeout() which destroys our custom busy handler.
+    Sqlite3.set_busy_timeout(db, Pragma.busy_timeout(options))
   end
 
   defp deserialize(db, options) do