mgr: use a struct for DaemonKey

mgr: use a struct for DaemonKey

instead of using `std::pair<>` for presenting it. there are three
advantages:

1. better names for readability: `type` and `name` are better names
   than `first` and `second`
2. so we can specliaze its behavior by its type: define
   `operator<<` once, and use it everywhere. no need to worry
   about ADL to pickup the generic operator of `operator<< (..., pair<>)`
   anymore. so we can safely use `<< key`, and no need to
   use `<< key.first << '.' << key.second`. kill the printed form of
   `osd,1` once and for all, all of them are now "osd.1".
3. consolidate the print/parse in a single place

Fixes: https://tracker.ceph.com/issues/42079
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit 5aac7eba36be99861eb8726872e339ff55360f47)

 Conflicts:
	src/mgr/DaemonServer.cc
	src/mgr/Mgr.cc
 - convert/edit std::pair to DaemonKey:name-value in a few places