Timer Wrapper Functions, and Refactoring

[Anotra]

What?

add DISCORD_TIMER_GET flag
add DISCORD_TIMER_INTERVAL_FIXED flag which causes a timer to run based upon it's original start time, instead of the current time
allow discord_timer_ctl to cancel timers
allow delay -1 to disable a timer after it has started

[Anotra]

What?

added wrapper functions to make timers easier to manage

bool discord_timer_get(struct discord *client, unsigned id,
                                      struct discord_timer *timer);
bool discord_timer_start(struct discord *client, unsigned id);
bool discord_timer_stop(struct discord *client, unsigned id);
bool discord_timer_cancel(struct discord *client, unsigned id);
bool discord_timer_delete(struct discord *client, unsigned id);
bool discord_timer_cancel_and_delete(struct discord *client, unsigned id);