CAS from couch_dbdump --json is incorrectly rounded to 2^53

couch_dbdump --json since MB-49817 (v7.1.0) outputs CAS as JSON number types.

While the JSON library we use (nlohmann::json) has no problem with 64bit integers, other JSON libraries / tools (such as jq) do not correctly parse these large numbers and silently discard least-significant bits / round the value. For example, note the difference in displayed CAS value with and without piping the output of couch_dbdump to jq :

install/bin/couch_dbdump --key  doc_key --json ../ns_server/data/n_0/data/default/997.couch.1 

{"body":{"json":"maybe?"},"cas":1664543368459911168, ... <cut>

vs

install/bin/couch_dbdump --key doc_key --json ../ns_server/data/n_0/data/default/997.couch.1 | jq -c

{"body":{"json":"maybe?"},"cas":1664543368459911200," ... <cut>

The correct CAS value of 1664543368459911168 is mangled and is rounded to 1664543368459911200.

In the specific case of jq this is a known bug and fixed in master, but is not fixed in the current latest release (1.6).

To try to mitigate these kinds of problems in future, we should change couch_dbdump to represent CAS values as stings which will not be mangled.