mds: use monotonic waits in Beacon

mds: use monotonic waits in Beacon

This guarantees that the sender thread cannot be disrupted by system clock
changes. This commit also simplifies the sender thread by manually managing the
thread and avoiding unnecessary context creation.

Fixes: http://tracker.ceph.com/issues/26962
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>
(cherry picked from commit a5fc29b95281c6ca58c9177c665c379846beb4b3)

Conflicts:
	src/mds/Beacon.cc
- g_conf->foo instead of g_conf()->foo
- boost::string_view instead of std::string_view
- always specify template type std::unique_lock<std::mutex>
	src/mds/Beacon.h
- time::min() instead of clock::zero()
- always specify template type std::unique_lock<std::mutex>
- std::chrono::seconds instead of "1s" in std::chrono_literals namespace
  (which is a C++14ism)