Rename ssl3_get_key_exchange to ssl3_get_server_key_exchange.

More consistent with ssl3_send_server_key_exchange and the message name.

Change-Id: If0f435a89bdf117297d349099708fff0bd5a6e98
Reviewed-on: https://boringssl-review.googlesource.com/1170
Reviewed-by: Adam Langley <agl@google.com>