MINOR: ssl: Use OCSP_CERTID instead of ckch_store in ckch_store_build_certid

MINOR: ssl: Use OCSP_CERTID instead of ckch_store in ckch_store_build_certid

The only useful information taken out of the ckch_store in order to copy
an OCSP certid into a buffer (later used as a key for entries in the
OCSP response tree) is the ocsp_certid field of the ckch_data structure.
We then don't need to pass a pointer to the full ckch_store to
ckch_store_build_certid or even any information related to the store
itself.
The ckch_store_build_certid is then converted into a helper function
that simply takes an OCSP_CERTID and converts it into a char buffer.