Allow for downloading new work before current work unit has completed

[thelomic]

v7 client would download a new work unit upon the current work unit reaching 99% (this was also a configurable percentage, with 99% being the default). I've noticed v8 doesn't seem to begin downloading a new WU until the current WU is completed and it has finished uploading.

This downtime between WU execution, while often fairly short, causes more thermal cycles on hardware rather than allowing it to remain in steady-state load most of the time. Assign servers also sometimes require a few retries before assigning new work, which creates unnecessary downtime.

Thanks for all the hard work on v8!