net: tls: Handle HELLO_VERIFY_REQUIRED during DTLS handshake

DTLS handshake can return MBEDTLS_ERR_SSL_HELLO_VERIFY_REQUIRED, which
indicate that TLS session context should be reset.

Also, store information whether TLS connection has beed established.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>