[Bug] Concurrent access _tablets_under_clone/restore should also sharding

[acelyc111]

Describe the bug
I found a coredump backtrace looks like:

Program terminated with signal 11, Segmentation fault.
#0  _M_lower_bound (this=<optimized out>, __k=<optimized out>, __y=0x1abc07c80, __x=0x20) at /usr/include/c++/7.3.0/bits/stl_tree.h:1872
1872	/usr/include/c++/7.3.0/bits/stl_tree.h: 没有那个文件或目录.
Missing separate debuginfos, use: debuginfo-install glibc-2.17-157.el7_3.1.x86_64 libgcc-4.8.5-28.el7_5.1.x86_64 zlib-1.2.7-17.el7.x86_64
(gdb) bt
#0  _M_lower_bound (this=<optimized out>, __k=<optimized out>, __y=0x1abc07c80, __x=0x20) at /usr/include/c++/7.3.0/bits/stl_tree.h:1872
#1  equal_range (__k=<synthetic pointer>, this=0x53d8130) at /usr/include/c++/7.3.0/bits/stl_tree.h:1951
#2  erase (__x=<synthetic pointer>, this=0x53d8130) at /usr/include/c++/7.3.0/bits/stl_tree.h:2500
#3  erase (__x=<synthetic pointer>, this=0x53d8130) at /usr/include/c++/7.3.0/bits/stl_set.h:675
#4  doris::TabletManager::unregister_clone_tablet (this=0x53d8000, tablet_id=3120130) at /builds/olap/doris/be/src/olap/tablet_manager.cpp:1105
#5  0x0000000001637bcb in doris::EngineCloneTask::execute (this=0x7f2f08b0abd0) at /builds/olap/doris/be/src/olap/task/engine_clone_task.cpp:69
#6  0x0000000000e11565 in doris::StorageEngine::execute_task (this=0x4df9c00, task=task@entry=0x7f2f08b0abd0) at /builds/olap/doris/be/src/olap/storage_engine.cpp:996
#7  0x0000000001464318 in doris::TaskWorkerPool::_clone_worker_thread_callback (this=0x2bb858fc0) at /builds/olap/doris/be/src/agent/task_worker_pool.cpp:872
#8  0x0000000001176e42 in operator() (this=0x2c23f2ad8) at /usr/include/c++/7.3.0/bits/std_function.h:706
#9  run (this=0x2c23f2ad0) at /builds/olap/doris/be/src/util/threadpool.cpp:42
#10 doris::ThreadPool::dispatch_thread (this=0x239eb90e0) at /builds/olap/doris/be/src/util/threadpool.cpp:551
#11 0x000000000116eca8 in operator() (this=0x2c23eab58) at /usr/include/c++/7.3.0/bits/std_function.h:706
#12 doris::Thread::supervise_thread (arg=0x2c23eab40) at /builds/olap/doris/be/src/util/thread.cpp:385
#13 0x00007f2f81fdedc5 in start_thread () from /lib64/libpthread.so.0
#14 0x00007f2f822ea73d in clone () from /lib64/libc.so.6
(gdb)

I should point out that one of my config is:

tablet_map_shard_size=256