feat(update_replication_factor#7): replace cluster-level max_replicas_in_group with max_replica_count of each replica

[empiredan]

This is a subtask of apache/incubator-pegasus#865.

There is an option max_replicas_in_group which restricts the max replica count of the whole cluster. It's a cluster-level option. However, now that it's allowed to update the replication factor of each table, this cluster-level option should be replaced.

Conceptually max_replicas_in_group is the total number of alive and dropped replicas. Its default value is 4. For a table that has replication factor 3, that max_replicas_in_group is set to 4 means 3 alive replicas plus a dropped replica.

max_replicas_in_group can also be loaded from configuration file, which means its default value will be overridden. The value of max_replicas_in_group will be assigned to another static variable MAX_REPLICA_COUNT_IN_GRROUP, whose default value is also 4.

For unit tests, MAX_REPLICA_COUNT_IN_GRROUP is set to the default value 4; for production environments, MAX_REPLICA_COUNT_IN_GRROUP is set to 3 since max_replicas_in_group is configured as 3 in .ini file.

Since the cluster-level option max_replicas_in_group contains the alive and dropped replicas, we can use the replication factor of each table as the number of alive replicas, and introduce another option
max_reserved_dropped_replicas representing the max reserved number allowed for dropped replicas.

If max_reserved_dropped_replicas is set to 1, there is at most one dropped replicas reserved; If it's set to 0, however, none of dropped replicas can be reserved.

Thus the default value of max_reserved_dropped_replicas is set to 1 to so that the unit tests can be passed.
For production environments, it should be set to 0 to be consistent with max_replicas_in_group, which also means, once the number of alive replicas reaches max_replica_count, at most one dropped replica can be reserved and others will be eliminated.

[meta_server]
+ max_reserved_dropped_replicas = 0

[acelyc111]

LGTM

[hycdong]

New configuration added:

[meta_server]
+ max_reserved_dropped_replicas = 1