Use hostname removing peer_cn.

* src/context.h (struct _ntbtls_context_s): Remove peer_cn field.
* src/protocol.c (_ntbtls_read_certificate): Use ->hostname.

This change reflects the change of mbedtls API:
bc2b771af4b67c900813e58e7c8c77d7907291c1

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>