rust: bssl-tls: Disallow downcasting in-handshake to normal handle

A big downside in doing so is that it accidentally allows users to
trigger progression in TLS state machine during the handshake.
This is possible because we supply various callbacks with the
in-handshake handle and through this existing downcasting the callbacks
can call read and write again, which is a re-entry into our handshake code.

Update-Note: a breaking change is introduced to further restrict the
`in_handshake()` handle to the connection, so that one cannot possibly
advance the TLS state machine while a read or write into the TLS socket
is in progress. This was possible through callbacks that can be invoked
while reading or writing on the socket. The necessary changes can be
found in cl/934247882.

Signed-off-by: Xiangfei Ding <xfding@google.com>
Change-Id: I3212b0184a3ebda606812217a4e350396a6a6964
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/97667
Presubmit-BoringSSL-Verified: boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com <boringssl-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Benjamin <davidben@google.com>