MEDIUM: config: permit to start a bind on multiple groups at once

MEDIUM: config: permit to start a bind on multiple groups at once

Now it's possible for a bind line to span multiple thread groups. When
this happens, the first one will become the reference and will be entirely
set up, and the subsequent ones will be duplicated from this reference,
so that they can be registered in distinct groups. The reference is
always setup and started first so it is always available when the other
ones are started.

The doc was updated to reflect this new possibility with its limitations
and impacts, and the differences with the "shards" option.