crimson/monc: fix send_message() racing with reopen_session().

crimson/monc: fix send_message() racing with reopen_session().

The `send_message()` method is a high-level facility for
communicating with a monitor. If there is an active conn
available, it sends the message immediately; otherwise
the message is queued. This method assumes the queue is
already drained if the connection is available.

`active_con` is managed by `reopen_session()` where it's
first cleared and then reset after finding new alive mon.
This is followed by draining the `pending_messages` queue
which happens in `on_session_opened()` after the `MAuth`
exchange is finished.

Unfortunately, the path from the `active_con` clearing
to draining the queue is long and divided into multiple
continuations which results in lack of atomicity. When
e.g. `run_command()` interleaves the stages, following
crash happens:

```
INFO  2021-05-07 08:13:43,914 [shard 0] monc - do_auth_single: mon v2:172.21.15.82:6805/34166 => v2:172.21.15.82:3300/0 returns auth_reply(proto 2 0 (0) Success) v1: 0
ceph-osd: /home/jenkins-build/build/workspace/ceph-dev-new-build/ARCH/x86_64/AVAILABLE_ARCH/x86_64/AVAILABLE_DIST/centos8/DIST/centos8/MACHINE_SIZE/gigantic/release/17.0.0-3910-g1b18e076/rpm/el8/BUILD/ceph-17.0.0-3910-g1b18e076/src/crimson/mon/MonClient.cc:1034: seastar::future<> crimson::mon::Client::send_message(MessageRef): Assertion `pending_messages.empty()' failed.
Aborting on shard 0.
Backtrace:
 0# 0x000055CDE6DB532F in ceph-osd
 1# FatalSignal::signaled(int, siginfo_t const*) in ceph-osd
 2# FatalSignal::install_oneshot_signal_handler<6>()::{lambda(int, siginfo_t*, void*)#1}::_FUN(int, siginfo_t*, void*) in ceph-osd
 3# 0x00007FC1BF20BB20 in /lib64/libpthread.so.0
 4# gsignal in /lib64/libc.so.6
 5# abort in /lib64/libc.so.6
 6# 0x00007FC1BD806B09 in /lib64/libc.so.6
 7# 0x00007FC1BD814DE6 in /lib64/libc.so.6
 8# crimson::mon::Client::send_message(boost::intrusive_ptr<Message>) in ceph-osd
 9# crimson::mon::Client::renew_subs() in ceph-osd
10# 0x000055CDE764FB0B in ceph-osd
11# 0x000055CDE10457F0 in ceph-osd
12# 0x000055CDEA0AB88F in ceph-osd
13# 0x000055CDEA0B0DD0 in ceph-osd
14# 0x000055CDEA2689FB in ceph-osd
15# 0x000055CDE9DC0EDA in ceph-osd
16# main in ceph-osd
17# __libc_start_main in /lib64/libc.so.6
18# _start in ceph-osd
```

The problem caused following failure at Sepia:
http://pulpito.front.sepia.ceph.com/rzarzynski-2021-05-07_07:41:02-rados-master-distro-basic-smithi/6104549

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>