BUG/MINOR: backend: balance uri specific options were lost across defaults

BUG/MINOR: backend: balance uri specific options were lost across defaults

The "balance uri" options "whole", "len" and "depth" were not properly
inherited from the defaults sections. In addition, "whole" and "len"
were not even reset when parsing "uri", meaning that 2 subsequent
"balance uri" statements would not have the expected effect as the
options from the first one would remain for the second one.

This may be backported to all maintained versions.