The DTLS stack has two very different APIs for handling timeouts. In
non-blocking mode, timeouts are driven externally by the caller with
DTLSv1_get_timeout. In blocking mode, timeouts are driven by the BIO by
calling a BIO_ctrl with BIO_CTRL_DGRAM_SET_NEXT_TIMEOUT.

The latter is never used by consumers, so remove support for it.
BIO_CTRL_DGRAM_SET_NEXT_TIMEOUT implicitly depends on struct timeval
being used for timestamps, which we would like to remove. Without this,
the only public API which relies on this is the testing-only
SSL_CTX_set_current_time_cb which is BoringSSL-only and we can change at
our leisure.

BUG=155

Change-Id: Ic68fa70afab2fa9e6286b84d010eac8ddc9d2ef4
Reviewed-on: https://boringssl-review.googlesource.com/13945
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>