librbd/migration/HttpClient: avoid reusing ssl_stream after shut down

librbd/migration/HttpClient: avoid reusing ssl_stream after shut down

ssl_stream objects can't be reused after shut down: despite
a successful reconnect and handshake, any attempt to read or write
fails with "end of stream" (beast.http:1) or "protocol is shutdown"
(asio.ssl:337690831) error respectively.  This doesn't appear to be
documented, but Beast and ASIO authors both mention that the stream
must be destroyed and recreated [1][2].

This was missed because the only integration test with a big enough
image used http instead of https.

[1] https://github.com/boostorg/beast/issues/821#issuecomment-338354949
[2] https://github.com/chriskohlhoff/asio/issues/804#issuecomment-872746894

Fixes: https://tracker.ceph.com/issues/69178
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>