crimson/monc: set name using a setter

crimson/monc: set name using a setter

* set entity_name using a setter not pass it to constructor, because
  the entity_name is retrieved in seastar's app.run() by ConfigProxy,
  while it'd be simpler if we can instantiate mon::Client in main()
  as a local variable, instead of managing it on heap using a smart
  pointer. so we cannot pass the entity_name as a parameter of ctor.
* also cleanup the #include's, as they are included already in the
  header.

Signed-off-by: Kefu Chai <kchai@redhat.com>