Implement RFC 3161 timestamp responses (#293)

* Add timestamps

Signed-off-by: Asra Ali <asraa@google.com>

* change

Signed-off-by: Asra Ali <asraa@google.com>

* address comments

Signed-off-by: Asra Ali <asraa@google.com>

* address comments

Signed-off-by: Asra Ali <asraa@google.com>

* fix binary writer

Signed-off-by: Asra Ali <asraa@google.com>

* add tsa

Signed-off-by: Asra Ali <asraa@google.com>

* distangle cert chain creation from new signer

Signed-off-by: Asra Ali <asraa@google.com>

* revert some now unncessary changes

Signed-off-by: Asra Ali <asraa@google.com>

* cert chain 404

Signed-off-by: Asra Ali <asraa@google.com>

* fix

Signed-off-by: Asra Ali <asraa@google.com>