restrict LogBuffer's size

[ylgrgyq]

Currently, Leader will write uncommitted logs to LogBuffer without any constraint. So if the cluster has some problems and leader can not commit any logs while it still receiving proposals from the end user, this leader can overflow it's LogBuffer then shutdown unexpectedly.

We can set a constratint like max uncommited logs, if uncommited logs over this threshold, leader can refuse subsequent proposals until uncommited logs count droped to a safe value.