Fix event loop thread might exit unexpectedly

[BewareMyPower]

Fixes #209

Motivation

When the event loop thread is started from ExecutorService::restart, there is a chance that io_service_.run(ec) returns immediately. In this case, the ClientConnection::resolver_ that was registered in the IO service could block forever in async_resolve method.

Modifications

In the event loop thread, if the io_service::run method is not stopped by ExecutorService::close, just call restart and run again to avoid the event loop thread exits unexpectedly.

Run the ConnectionFailTest independently with --gtest_repeat=20 to avoid it's still flaky.

[BewareMyPower]

The threads info before this patch when it's stuck:

(gdb) info threads
  Id   Target Id                                          Frame
* 1    Thread 0x7fb2859c50c0 (LWP 7394) "ConnectionFailT" __futex_abstimed_wait_common64 (private=0, cancel=true, abstime=0x0, op=393, expected=0,
    futex_word=0x7fb270038ba0) at ./nptl/futex-internal.c:57
  2    Thread 0x7fb2851b8640 (LWP 7651) "ConnectionFailT" __futex_abstimed_wait_common64 (private=0, cancel=true, abstime=0x0, op=393, expected=0,
    futex_word=0x55de8e4853c8) at ./nptl/futex-internal.c:57
(gdb) thread 1
[Switching to thread 1 (Thread 0x7fb2859c50c0 (LWP 7394))]
#0  __futex_abstimed_wait_common64 (private=0, cancel=true, abstime=0x0, op=393, expected=0, futex_word=0x7fb270038ba0) at ./nptl/futex-internal.c:57
57      in ./nptl/futex-internal.c
(gdb) bt
<...>
#5  0x00007fb2874fab99 in pulsar::Future<pulsar::Result, pulsar::Producer>::get (this=0x7ffe339bc330, result=...) at /app/lib/Future.h:69
#6  0x00007fb2874f8892 in pulsar::Client::createProducer (this=0x7fb2700372c8, topic="test-connection-fail-51678865999", conf=..., producer=...)
    at /app/lib/Client.cc:55
<...>
(gdb) thread 2
[Switching to thread 2 (Thread 0x7fb2851b8640 (LWP 7651))]
#0  __futex_abstimed_wait_common64 (private=0, cancel=true, abstime=0x0, op=393, expected=0, futex_word=0x55de8e4853c8) at ./nptl/futex-internal.c:57
57      in ./nptl/futex-internal.c
(gdb) bt
#0  __futex_abstimed_wait_common64 (private=0, cancel=true, abstime=0x0, op=393, expected=0, futex_word=0x55de8e4853c8) at ./nptl/futex-internal.c:57
#1  __futex_abstimed_wait_common (cancel=true, private=0, abstime=0x0, clockid=0, expected=0, futex_word=0x55de8e4853c8) at ./nptl/futex-internal.c:87
#2  __GI___futex_abstimed_wait_cancelable64 (futex_word=futex_word@entry=0x55de8e4853c8, expected=expected@entry=0, clockid=clockid@entry=0,
    abstime=abstime@entry=0x0, private=private@entry=0) at ./nptl/futex-internal.c:139
#3  0x00007fb286c2fac1 in __pthread_cond_wait_common (abstime=0x0, clockid=0, mutex=0x55de8e485368, cond=0x55de8e4853a0) at ./nptl/pthread_cond_wait.c:503
#4  ___pthread_cond_wait (cond=0x55de8e4853a0, mutex=0x55de8e485368) at ./nptl/pthread_cond_wait.c:627
#5  0x00007fb28752be08 in boost::asio::detail::posix_event::wait<boost::asio::detail::conditionally_enabled_mutex::scoped_lock> (this=0x55de8e4853a0, lock=...)
    at /usr/include/boost/asio/detail/posix_event.hpp:119
#6  0x00007fb28751e700 in boost::asio::detail::conditionally_enabled_event::wait (this=0x55de8e485398, lock=...)
    at /usr/include/boost/asio/detail/conditionally_enabled_event.hpp:97
#7  0x00007fb28752035d in boost::asio::detail::scheduler::do_run_one (this=0x55de8e485330, lock=..., this_thread=..., ec=...)
    at /usr/include/boost/asio/detail/impl/scheduler.ipp:490
#8  0x00007fb28751fed6 in boost::asio::detail::scheduler::run (this=0x55de8e485330, ec=...) at /usr/include/boost/asio/detail/impl/scheduler.ipp:204
#9  0x00007fb287523903 in boost::asio::detail::resolver_service_base::work_scheduler_runner::operator() (this=0x55de8e499508)
    at /usr/include/boost/asio/detail/impl/resolver_service_base.ipp:38
#10 0x00007fb28758de66 in boost::asio::detail::posix_thread::func<boost::asio::detail::resolver_service_base::work_scheduler_runner>::run (this=0x55de8e499500)
    at /usr/include/boost/asio/detail/posix_thread.hpp:86
#11 0x00007fb28751e908 in boost::asio::detail::boost_asio_detail_posix_thread_function (arg=0x55de8e499500)
    at /usr/include/boost/asio/detail/impl/posix_thread.ipp:74
#12 0x00007fb286c30b43 in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:442
#13 0x00007fb286cc1bb4 in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:100