wdlc cannot be invoked parallely due to colliding output descriptor name

[mrjerryjohns]

When wdlc is invoked in parallel to compile different pieces of schema simultaneously, it fails due to all invocations writing to/deleting the same descriptor binary that gets generated as an intermediate artifact.

Unless specified by the user through the --output-intermediate FILE option, it should by default generate a randomized file name to reduce/omit the chance of collisions across parallel runs.