MINOR: ssl: Limit ocsp_uri buffer size to minimum

MINOR: ssl: Limit ocsp_uri buffer size to minimum

The ocsp_uri field of the certificate_ocsp structure was a 16k buffer
when it could be hand allocated to just the required size to store the
OCSP uri. This field is now behaving the same way as the sctl and
ocsp_response buffers of the ckch_store structure.