Memory check before checkpoint is inaccurate

[morningman]

Currently, Doris will check whether the jvm memory is enough to hold the checkpoint image. But the algorithm is inaccurate, it only concern the OlapTable objects. The Views, load jobs and other metadata are not included.
And inaccurate memory checking may cause OOM when doing checkpoint.

So now, I will use JVM old mem pool used percent to check the memory usage. And add to new FE config

metadata_checkopoint_memory_threshold. default is 60(%), if the mem pool used percent exceed this percent, checkpoint will not work.

force_do_metadata_checkpoint. default is false. In case the auto memory check has some problem, we can switch this config to do the checkpoint forcibly, ignore the memory usage.