Don't do handshake in mbedtls_ssl_read and mbedtls_ssl_write

[gilles-peskine-arm]

Don't perform handshake steps inside mbedtls_ssl_read() or mbedtls_ssl_write(). Instead, have the user call mbedtls_ssl_handshake().

The goal is to simplify these functions and reduce the errors (especially “non-error errors” like MBEDTLS_SSL_WANT_READ and MBEDTLS_SSL_WANT_WRITE).

I am personally not convinced by this, especially if we do #9516. It wouldn't simplify the code that much: we can have read() be do { do_read(); if (handshake_needed) handshake_step(); } while (more_handshake_needed()).

[gilles-peskine-arm]

Having chatted with @mpg, whose idea I believe this originally was: the main intent is to reduce the complexity on application handling of non-error errors. #9516 solves this problem better, solves more and is not more work. So let's focus our effort on #9516 and remove this from consideration.