Add a simple dumb webserver that responds with the session status for
any GET request. This option is intended to be used with -loop to
generate automated responses to requests and serves two purposes: (1)
test that application data from clients can be decrypted, (2) test that
clients can decrypt data from the server and (3) early data indicator.

Change-Id: I2b8374ca7b8db4c8effab42e86b5e3139d9466e1
Reviewed-on: https://boringssl-review.googlesource.com/20305
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>