OPTIM/MINOR: server: remove unnecessary server lookup during purge

OPTIM/MINOR: server: remove unnecessary server lookup during purge

Function srv_check_for_deletion() was previously used in proxy purge
task to determine if a server is ready to be deleted. However, this
performs an unnecessary server lookup as the server instance is already
known at this stage.

Simplify this by replacing it with new helper srv_in_use(), which
reports if a server still has active/pending/idle connections. If not
the case, removal can be performed.

This code is safe as it runs in thread isolation. If a server is removed
via another path before proxy purge task, it will be removed from
backend purge list. Furthermore, it also eliminates unnecessary checks
on server configuration : only a removable server can be inserted in the
proxy purge list.