perf: enhance the loading process of replicas particularly when a significant number of replicas are spread across multiple disks

[empiredan]

Immediately after the replica server is started, all of the replicas under the data
directory would be loaded. Currently, a loading task is launched for each replica
directory. The tasks for loading replica directories are pushed into a partitioned
thread pool (namely THREAD_POOL_REPLICATION) disk by disk: only after all
of the replica directories on the current disk have been pushed into the thread
pool would the process move on to the next disk. Since the thread pool is partitioned
while the hash for each task is the total number of the tasks that have been in the
pool before this task is added, all of the replica directories on one disk would be
executed concurrently. This would lead to two problems once there are a great number
of replica directories on each disk:

• I/O usage for each disk might become saturated: its %util might become 100%;
• The entire loading process is blocked on each single disk: during a long period only
  one disk is keeping busy while others are idle.

The replica server seems getting stuck in loading replicas after it is started. This is
unacceptable and should be changed.

The improved version allows the replica directories on different disks to be loaded
simultaneously: every disk would be busy loading replicas. Also, loading tasks would
be pushed into a non-partitioned thread pool (i.e. THREAD_POOL_LOCAL_APP)
instead of the partitioned, making tasks across multiple threads auto-balanced to
prevent some threads from being starved while others are stuffed.

And new parameter is added to restrict the max number of replicas allowed to be loaded
simultaneously for each disk, in case that I/O usage for each disk becomes saturated.
Another parameter is added to ensure that the main thread waiting all loading tasks to
finished would not be blocked on one task too long while the number of tasks for loading
replica directories simultaneously on a single disk has reached its limit.

Parameters are added as follows:

[replication]
+ max_replicas_on_load_for_each_disk = 256
+ load_replica_max_wait_time_ms = 10