ews: fix an ABA lockup involving EWS unsubscribe actions

There is a ABA deadlock occurring:

* T83/P11041 holding dbase<user/7/4>.giant_lock
* T83/P11041 waiting on EWSPlugin::subscriptionLock<WR> here:

```
f4  std::unique_lock<std::mutex>::lock (this=this@entry=0x7f148dffbd10) at /usr/include/c++/12/bits/unique_lock.h:139
f5  std::unique_lock<std::mutex>::unique_lock (__m=..., this=0x7f148dffbd10) at /usr/include/c++/12/bits/unique_lock.h:69
f6  gromox::EWS::EWSPlugin::event (this=0x75b860, dir=<optimized out>, ID=627, notification=0x7f13fc571a98) at exch/ews/ews.cpp:812
f7  ews_event_proc (dir=<optimized out>, table=<optimized out>, ID=<optimized out>, notification=<optimized out>) at exch/ews/ews.cpp:761
f8  exmdb_server::event_proc (dir=0x7f1484cc0270 "/var/lib/gromox/user/7/4", is_table=0, notify_id=627, datagram=datagram@entry=0x7f13fc571a98) at exch/exmdb/server.cpp:133
f9  notification_agent_backward_notify (remote_id=<optimized out>, pnotify=pnotify@entry=0x7f13fc571a70) at exch/exmdb/notification_agent.cpp:22
f10 dg_notify (notifq=...) at exch/exmdb/db_engine.cpp:574
f11 exmdb_server::write_message_v2 (dir=<optimized out>, cpid=cpid@entry=CP_ACP, folder_id=<optimized out>, pmsgctnt=0x7f1484e2eba0, outmid=outmid@entry=0x7f148dffc3a8, outcn=outcn@entry=0x7f148dffc3b0, pe_result=0x7f1484bd20ac) at exch/exmdb/message.cpp:3851
f12 exmdb_server::write_message (dir=dir@entry=0x7f1484cc0270 "/var/lib/gromox/user/7/4", cpid=cpid@entry=CP_ACP, folder_id=folder_id@entry=15137215173615419393, ctnt=ctnt@entry=0x7f1484e2eba0, e_result=e_result@entry=0x7f1484bd20ac) at exch/exmdb/message.cpp:3864
f13 exmdb_server::flush_instance (dir=0x7f1484cc0270 "/var/lib/gromox/user/7/4", instance_id=<optimized out>, pe_result=0x7f1484bd20ac) at exch/exmdb/instance.cpp:1518
```

* T72/P10387 holding EWSPlugin::subscriptionLock.
* T72/P10387 waiting on dbase<user/7/4>.giant_lock here:

```
f3  std::shared_mutex::lock (this=<optimized out>) at /usr/include/c++/12/shared_mutex:420
f4  db_conn::lock_base_wr (this=this@entry=0x7f149a381790) at exch/exmdb/db_engine.cpp:509
f5  exmdb_server::unsubscribe_notification (dir=dir@entry=0x7f1460e6bbf0 "/var/lib/gromox/user/7/4", sub_id=<optimized out>, sub_id@entry=629) at /usr/include/c++/12/optional:306
f6  exmdb_client_local::unsubscribe_notification (dir=0x7f1460e6bbf0 "/var/lib/gromox/user/7/4", sub_id=629) at exch/exmdb/rpc.cpp:1681
f7  gromox::EWS::EWSPlugin::unsubscribe (this=0x75b860, key={...}) at exch/ews/ews.cpp:1104
f8  gromox::EWS::EWSPlugin::Subscription::~Subscription (this=0x7f1460964a90, __in_chrg=<optimized out>) at exch/ews/ews.cpp:794
 ...
f16 std::shared_ptr<gromox::EWS::EWSPlugin::Subscription>::~shared_ptr (this=0x7f149a3818a0, __in_chrg=<optimized out>) at /usr/include/c++/12/bits/shared_ptr.h:175
f17 gromox::EWS::EWSPlugin::unsubscribe (this=0x75b860, subscriptionKey=<optimized out>, username=0x7f149e59977c "[email protected]") at exch/ews/ews.cpp:1091
f18 gromox::EWS::EWSContext::unsubscribe (this=this@entry=0x7f14630fc860, subscriptionId=...) at exch/ews/context.cpp:2204
f19 gromox::EWS::EWSContext::~EWSContext (this=this@entry=0x7f14630fc860, __in_chrg=<optimized out>) at exch/ews/context.cpp:342
 ...
f22 std::unique_ptr<gromox::EWS::EWSContext, std::default_delete<gromox::EWS::EWSContext> >::reset (__p=0x0, this=<optimized out>) at /usr/include/c++/12/bits/unique_ptr.h:501
f23 gromox::EWS::EWSPlugin::term (this=<optimized out>, ctx=<optimized out>) at exch/ews/ews.cpp:755
f24 operator() (ctx=<optimized out>, __closure=0x0) at exch/ews/ews.cpp:593
f25 _FUN () at exch/ews/ews.cpp:593
f26 hpm_processor_insert_ctx (phttp=phttp@entry=0x7f149e599338) at exch/http/hpm_processor.cpp:500
f27 http_end (ctx=ctx@entry=0x7f149e599338) at exch/http/http_parser.cpp:489
f28 http_parser_process (vcontext=0x7f149e599338) at exch/http/http_parser.cpp:2123
```

EWSPlugin::SubManager::inner_subs does not need protection
(especially not by foreign lock EWSPlugin::subscriptionLock) because
it is only ever touched during creation (in ``make_submgr()``) and
then again in the destructor.

References: DESK-2280 DESK-2535

exch/ews/ews.cpp

@@ -790,7 +790,6 @@ EWSPlugin::SubManager::SubManager(const char *uname, const EWSPlugin &plugin) :
  */
 EWSPlugin::SubManager::~SubManager()
 {
-	std::lock_guard ss_lock(ews.subscriptionLock);
 	for (const auto &subKey : inner_subs)
 		ews.unsubscribe(subKey);
 	if(waitingContext)