Make last_seen_ts_threshold for getting alive executor at the scheduler side larger than the heartbeat time interval

[yahoNanJing]

Which issue does this PR close?

Closes #785.

Rationale for this change

What changes are included in this PR?

• Make hard coded settings configurable, like DEFAULT_EXECUTOR_TIMEOUT_SECONDS, EXPIRE_DEAD_EXECUTOR_INTERVAL_SECS.
• Make several structs for scheduler have the SchedulerConfig property and remove detailed settings, like ExecutorManager, QueryStageScheduler, etc.
• Use the executor_timeout_seconds for alive executor detection. It's larger than the executor_heartbeat_interval_seconds.

Are there any user-facing changes?

[yahoNanJing]

Hi @thinkharderdev and @andygrove, could you help review this PR?