… server restarts

Problem
-------

Server restart will trigger Socket::SetFailed -> Socket check healthy
-> Socket::WaitAndReset -> Reconnect to server -> Socket::Revive
on the brpc client side

Socket::Revive
-> _versioned_ref.cas(vref, <id_ver, nref + 1>) // step 1
   -> _recycle_flag = false // step 2

After step 1 is successfully executed, the Socket can be acquired by Socket::Address
and the gap between 1 and 2 will cause the ref count  in _versioned_ref to not be
decreased in Socket::ReleaseAdditionalReference since _recycle_flag is true
Finally Socket::WaitAndReset can never wait for the expected ref count to mistakenly
think that there are still another requests holding socket references
and fall into an infinite loop

Example:

Session1(Thread1)                                   Session2(Thread2)

Socket::Revive
T1: _versioned_ref.cas(vref, <id_ver, nref + 1>)
                                                    T2: Socker::Address success
                                                    T3: Server restart or network error happened
                                                    T4: Socket::SetFailed
                                                    T5: Socket::ReleaseAdditionalReference
// Now _recycle_flag is true                        T6: _recycle_flag.cas(false, true)
                                                        -> Failed: No call Socket::Dereference
T7: _recycle_flag = false
                                                    T8: Socket::WaitAndReset
                                                        -> nref always be three, not excepted two
                                                        -> dead loop. cannot reconnected to server

Solution
--------
Add _recycle_mutex to avoid this race condition

Socket::Revive and Socket::ReleaseAdditionalReference will only be called
when some exceptions occur in the socket, so this mutex will not cause
performance degradation