TimerLoop crashes on SIGINT

```
stack backtrace:
0: 0x5620918d8876 - backtrace::backtrace::libunwind::trace::h89ce6f97ed704bb8
at /home/sgkim126/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.7/src/backtrace/libunwind.rs:53
- backtrace::backtrace::trace::h2ea11176de8b52b9
at /home/sgkim126/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.7/src/backtrace/mod.rs:42
1: 0x5620918d2d13 - backtrace::capture::Backtrace::new_unresolved::h12c4aafe92d9af95
at /home/sgkim126/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.7/src/capture.rs:88
2: 0x5620918d2c6d - backtrace::capture::Backtrace::new::hc701411efe5ca320
at /home/sgkim126/.cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.7/src/capture.rs:63
3: 0x5620906d13c3 - panic_hook::panic_hook::hbb6739b4e365caf1
at util/panic_hook/src/lib.rs:53
4: 0x5620906d19e7 - core::ops::function::Fn::call::h2dc66d0d14de4fcb
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libcore/ops/function.rs:69
5: 0x5620922c1ce8 - rust_panic_with_hook
at src/libstd/panicking.rs:482
6: 0x5620922c1781 - continue_panic_fmt
at src/libstd/panicking.rs:385
7: 0x5620922c1665 - rust_begin_unwind
8: 0x5620922df57c - panic_fmt
at src/libcore/panicking.rs:85
9: 0x562090add354 - core::result::unwrap_failed::h2817faf90ff1a575
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libcore/macros.rs:17
10: 0x562090abe15a - >::expect::h64de81945013c413
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libcore/result.rs:825
11: 0x562090a282bd - ::set_min_timer::h0eb20439134cfd92
at core/src/client/client.rs:363
12: 0x562090a5f291 - ::chain_new_blocks::hd6a80edf853db3c4
at core/src/miner/miner.rs:732
13: 0x562090e51bf9 - codechain_core::client::importer::Importer::import_verified_blocks::ha5a3854c5eda42e2
at core/src/client/importer.rs:150
14: 0x562090a264fe - codechain_core::client::client::Client::import_verified_blocks::h35d8db9a0bf8ccd1
at core/src/client/client.rs:213
15: 0x562090af2956 - >::message::h53ccf91adf84b0ae
at core/src/service.rs:95
16: 0x562090a74551 - >::send_sync::h459884613492884a
at /home/sgkim126/workspace/kodebox/codechain/util/io/src/service.rs:399
17: 0x562090e13d99 - codechain_core::verification::queue::QueueSignal::set_sync::h0a42193f9c314458
at core/src/verification/queue/mod.rs:97
18: 0x562090b2aeaa - >::verify::h7b47507c00bd1d60
at core/src/verification/queue/mod.rs:297
19: 0x562090b28cd1 - >::new::{{closure}}::h9862f706e4923889
at core/src/verification/queue/mod.rs:167
20: 0x562090e62def - std::sys_common::backtrace::__rust_begin_short_backtrace::h1bf39baf982c5bbf
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libstd/sys_common/backtrace.rs:135
21: 0x562090dc0782 - std::thread::Builder::spawn_unchecked::{{closure}}::{{closure}}::hb3fe9523bd77671e
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libstd/thread/mod.rs:469
22: 0x562090e70d22 - as core::ops::function::FnOnce<()>>::call_once::h29ab5b4ba66ddebe
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libstd/panic.rs:309
23: 0x562090e72186 - std::panicking::try::do_call::h12bb562d37ded902
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libstd/panicking.rs:297
24: 0x5620922cb499 - __rust_maybe_catch_panic
at src/libpanic_unwind/lib.rs:87
25: 0x562090e71a2f - std::panicking::try::h326c2a89544fc7aa
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libstd/panicking.rs:276
26: 0x562090e70e62 - std::panic::catch_unwind::h41782e7179ae43cd
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libstd/panic.rs:388
27: 0x562090dbf35d - std::thread::Builder::spawn_unchecked::{{closure}}::h2b77fd9e35fb8566
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libstd/thread/mod.rs:468
28: 0x562090dc0c47 - >::call_box::h42601d6e6d8f3c55
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/liballoc/boxed.rs:749
29: 0x5620922ca80d - call_once<(),()>
at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/liballoc/boxed.rs:759
- start_thread
at src/libstd/sys_common/thread.rs:14
- thread_start
at src/libstd/sys/unix/thread.rs:81
30: 0x7f1ceb3bd6da - start_thread
31: 0x7f1ceaece88e - __clone
32: 0x0 -

Thread 'Verifier #2' panicked at 'Reseal min timer clear succeeds: TimerLoopDropped', src/libcore/result.rs:997

This is a bug. Please report it at:

https://github.com/CodeChain-io/codechain/issues/new
```