job-list: limit size of job constraint

[chu11]

As @grondo notes in #5656, "very large" constraints might have the ability to hang (or minimally slow down) the job-list service.

We may wish to put a limit on the size of job constraints to limit this.

Without thinking about this that much, perhaps a cap on the length of an operator array might be worthwhile. 32 or 64? i.e.

{ "and": [ { "userid": [ ... ]}, { "name": [ ...] }, ... ] }`

If we say there's a cap of 32, the max length of the values array for "and" would be 32, and the max of "userid" and "name" would also be 32.

Also we'd probably need a "recursive" limit too, without thinking about it much ... 8 or 16?