calibrate upload rate against availability

[r3cgm]

Perform a bandwidth test to see what the upload rate currently is, e.g. 6mbit/s. Take that result and use it to calculate some fractional upload rate that rsync can use, such as 85%.

Also consider a dependent feature where the rsync is periodically terminated and restarted to allow for recalibration of the upload rate. Since partial transfers are preserved, this should not harm the file upload in progress. And this would help with the use case where local non-backup network needs might increase, at the same time that a long sustained backup was in progress and taking all that bandwidth.

[r3cgm]

When performing a bandwidth calibration test, don't forget to use pseudorandom binary data in order to compensate for compression, and provide a 'true' estimate of bandwidth available.