MEDIUM: log/balance: merge tcp/http algo with log ones

MEDIUM: log/balance: merge tcp/http algo with log ones

"log-balance" directive was recently introduced to configure the
balancing algorithm to use when in a log backend. However, it is
confusing and it causes issues when used in default section.

In this patch, we take another approach: first we remove the
"log-balance" directive, and instead we rely on existing "balance"
directive to configure log load balancing in log backend.

Some algorithms such as roundrobin can be used as-is in a log backend,
and for log-only algorithms, they are implemented as "log-$name" inside
the "backend" directive.

The documentation was updated accordingly.