merge revision(s) f91879a7,95570692: [Backport #15992]

	handle_interrupt to defend monitor state [Bug #15992]

	If an exception is raised from another thread for example Timeout
	and this thread is just after `mon_exit`'s `@mon_owner = nil`,
	the exception breaks the state of MonitorMixin. To prevent that situation,
	it need to block interruption in mon_enter and mon_exit.

	Avoid creating Hash objects per each mon_synchronize call (#2393)


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67774 b2dd03c8-39d4-4d8f-98ff-823fe69b080e