(#21427) Do not send arrays as YAML in query params

Previously, when the REST requests were constructed and one of the
parameters was an Array, the Array was encoded as YAML and then
identified on the master and parsed as YAML. This was used specifically
to send the list of "ignores" when making file metadata requests.

This changes the system to now use multi-valued query parameters instead
of YAML. Both the agent and master have been updated to support this,
however existing masters will not be able to understand the new requests
correctly since they will either:

• in webrick only choose the first value as the value of the parameter
• in rack fail because the decode_params method does not understand arrays as values