This helps drive async callbacks from outside tokio-boring, such as in quiche.
Not a breaking change because every public item in tokio-boring is preserved as is.
Setting callbacks multiple times on a SslContextBuilder causes the previous callback
installed to leak, using replace_ex_data internally prevents that.
We also start using it in tokio-boring in with_ex_data_future, my understanding
is that the futures currently in use are never installed twice by that function
but that could change in the future with the addition of more async callbacks.
We introduce new methods replace_ex_data for both SslContextBuilder
and Ssl in case anyone is relying on the leaking behaviour of their
set_ex_data methods, but we do document that they leak now.
These two new kinds of methods immediately return a MidHandshakeSslStream
instead of actually initiating a handshake. This greatly simplifies
loops around MidHandshakeSslStream::WouldBlock.
`tokio_boring::HandshakeError` currently requires that the inner stream
implements `Debug` for its `Display` implementation. This constraint is
unnecessary.
This change removes this `Debug` constraint so `HandshakeError` always
implements `Display`.
Kevin Guthrie