MINOR: server: make use of srv_is_usable() instead of checking eweight

MINOR: server: make use of srv_is_usable() instead of checking eweight

srv_is_usable() is broader than srv_is_usable() as it not only considers
the weight but the server's state as well. Future changes will allow a
server to be in drain mode with a non-zero weight, so we should migrate
to use that function instead.