Bug: coredump in function last_prepared_decree()

## Bug Report
What version of Pegasus are you using?
`Pegasus Server 1.12.1 (694cbd544436f03d34bfbfcbca0cd9b8f397197e)`

Coredump Stack
```
(gdb) bt
#0 0x00007f571187e1d7 in raise () from /lib64/libc.so.6
#1 0x00007f571187f8c8 in abort () from /lib64/libc.so.6
#2 0x00007f5711877146 in __assert_fail_base () from /lib64/libc.so.6
#3 0x00007f57118771f2 in __assert_fail () from /lib64/libc.so.6
#4 0x00007f57155dc98f in dsn::ref_counter::release_ref (this=)
at /home/wutao1/pegasus-release/rdsn/include/dsn/utility/autoref_ptr.h:71
#5 0x00007f57156bef1c in release_ref (this=)
at /home/wutao1/pegasus-release/rdsn/src/dist/replication/lib/replica.cpp:338
#6 ~ref_ptr (this=0x7f56cc5e2fc0, __in_chrg=)
at /home/wutao1/pegasus-release/rdsn/include/dsn/utility/autoref_ptr.h:142
#7 dsn::replication::replica::last_prepared_decree (this=this@entry=0x14187200)
at /home/wutao1/pegasus-release/rdsn/src/dist/replication/lib/replica.cpp:336
#8 0x00007f57156ff742 in dsn::replication::replica::on_copy_remote_state_completed(dsn::error_code, unsigned long, unsigned long, dsn::replication::learn_request&&, dsn::replication::learn_response&&) (this=0x14187200, err=...,
size=0, copy_start_time=152,
req=,
resp=) at /home/wutao1/pegasus-release/rdsn/src/dist/replication/lib/replica_learn.cpp:913
#9 0x00007f57158208a9 in dsn::task::exec_internal (this=this@entry=0x789b277af)
at /home/wutao1/pegasus-release/rdsn/src/core/core/task.cpp:180
---Type to continue, or q to quit---
#10 0x00007f5715834a6d in dsn::task_worker::loop (this=0x1da73f0)
at /home/wutao1/pegasus-release/rdsn/src/core/core/task_worker.cpp:211
#11 0x00007f5715834c39 in dsn::task_worker::run_internal (this=0x1da73f0)
at /home/wutao1/pegasus-release/rdsn/src/core/core/task_worker.cpp:191
#12 0x00007f57121d6600 in std::(anonymous namespace)::execute_native_thread_routine (__p=)
at /home/qinzuoyan/git.xiaomi/pegasus/toolchain/objdir/../gcc-4.8.2/libstdc++-v3/src/c++11/thread.cc:84
#13 0x00007f5712d3bdc5 in start_thread () from /lib64/libpthread.so.0
#14 0x00007f571194073d in clone () from /lib64/libc.so.6
```

Information from frame 7:
replica gpid is 10.55
mutation is 10.55.34.194880435
start decree is 194880434

Information from frame 6:
autoref_ptr is dsn::ref_ptr * const
mutation is 10.55.34.194880435

Corresponding code:
https://github.com/XiaoMi/rdsn/blob/4b14d3adeec056736d621645c15d2ef0e495b97e/src/dist/replication/lib/replica.cpp#L325-L338

https://github.com/XiaoMi/rdsn/blob/4b14d3adeec056736d621645c15d2ef0e495b97e/include/dsn/utility/autoref_ptr.h#L68-L71

This bug seems to caused by trying to release reference the mutation structure who has already been released.

Start with replica.cpp lines 325-338 and autoref_ptr.h lines 68-71, then trace the call from replica_learn.cpp line 913. Use the reported gpid, mutation, and start decree to understand the reference-release path. Done means the reported last_prepared_decree() path no longer aborts while releasing the mutation reference.

Written by the indexing model from the issue text.