Give SSL_SESSION a destructor.

Previously we'd partially attempted the ssl_st / bssl::SSLConnection
subclassing split, but that gets messy when we actually try to add a
destructor, because CRYPTO_EX_DATA's cleanup function needs an ssl_st*,
not a bssl::SSLConnection*. Downcasting is technically undefined at this
point and will likely offend some CFI-like check.

Moreover, it appears that even with today's subclassing split,
New<SSL>() emits symbols like:

W ssl_st*& std::forward<ssl_st*&>(std::remove_reference<ssl_st*&>::type&)

The compiler does not bother emitting them in optimized builds, but it
does suggest we can't really avoid claiming the ssl_st type name at the
symbol level, short of doing reinterpret_casts at all API boundaries.
And, of course, we've already long claimed it at the #include level.

So I've just left this defining directly on ssl_session_st. The cost is
we need to write some silly "bssl::" prefixes in the headers, but so it
goes. In the likely event we change our minds again, we can always
revise this.

Change-Id: Ieb429e8eaabe7c2961ef7f8d9234fb71f19a5e2a
Reviewed-on: https://boringssl-review.googlesource.com/29587
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>

ssl/ssl_cert.cc

diff --git a/ssl/ssl_cert.cc b/ssl/ssl_cert.cc
index 8b869c8..04c77ad 100644
--- a/ssl/ssl_cert.cc
+++ b/ssl/ssl_cert.cc
@@ -808,7 +808,7 @@
     return NULL;
   }
 
-  return session->certs;
+  return session->certs.get();
 }
 
 const STACK_OF(CRYPTO_BUFFER) *SSL_get0_server_requested_CAs(const SSL *ssl) {