crimson/mgr/client:Introduce Client::send()

crimson/mgr/client:Introduce Client::send()

Client::reconnect nullifies the connection used by the mgr client
before setting a new one.

In this time, we might re-use the nullptr connection due to tasks
that are being run in the background (See: dispatch_in_background).

To avoid this, we had multiple `if (!conn)` checks, some methods
even checked this condition twice to reduce the possibilty of using
undefined the connection.

Instead of introducing an additional check in Client::_send_report,
Introduce Client::send which would be responsible for:
a) Veryfing the connection is set
b) Trying to get a shared access to conn_lock

Client::reconnect will lock conn_lock exclusivly until the
connection is set. If we send is called while reconnecting,
sending will be dropped - same as before.

Fixes: https://tracker.ceph.com/issues/70179
Signed-off-by: Matan Breizman <mbreizma@redhat.com>