Add the scheduler option HeartbeatInterval

[pior]

Context

From https://github.com/hibiken/asynq/issues/510:

Currently the tick interval of Scheduler.runHeartbeater() and the ttl of WriteSchedulerEntries() are both 5*time.Second scheduler entries is prone to expire when redis is slow to respond

Redo of https://github.com/hibiken/asynq/pull/896

Fixes https://github.com/hibiken/asynq/issues/510

Changes

• Add option SchedulerOpts.HeartbeatInterval
• Change default scheduler heartbeat interval from 5s to 10s