CXXCBC-119: Set value of subdoc exists to "true" or "false" if result is success or path-not-found

[DemetrisChr]

Motivation

Path not found errors were cleared in #444. In addition to that, the value of the spec result should be set to the JSON representation of the exists field if the result was success or path-not-found. Relevant section of the RFC:

If the result came from an exists lookup, calls Exists to get a boolean, and uses the JSON representation of that boolean as the "value" in the next step. Propagates any exceptions thrown by Exists.

Changes

• Set value to the JSON encoding of the exists boolean for exists spec results
• Clear the path_not_found error in lookup_in_replica_request::make_response, to avoid having to duplicate the logic for lookup_in_any_replica and lookup_in_all_replicas. Remove the checks for a path_not_found error code everywhere that's no longer necessary because of this change.
• Update the tests to reflect the change in the value of an exists subdoc result

Example

const std::string document_id{ "minimal_example" };
const tao::json::value basic_doc{
    { "a", 1.0 },
    { "b", 2.0 },
};
collection.upsert(document_id, basic_doc).get();

auto specs = couchbase::lookup_in_specs{
    couchbase::lookup_in_specs::exists("a"),
    couchbase::lookup_in_specs::exists("c")
};
auto [ctx, resp] = collection.lookup_in("minimal_example", specs).get();
std::cout << "Spec 0 - content (as bool): " << resp.content_as<bool>(0) << ", exists: " << resp.exists(0) << "\n";
std::cout << "Spec 1 - content (as bool): " << resp.content_as<bool>(1) << ", exists: " << resp.exists(1) << "\n";

will output

Spec 0 - content (as bool): 1, exists: 1
Spec 1 - content (as bool): 0, exists: 0