[SDK] Fix potential race with NSKeyValueObservation
[SDK] Fix potential race with NSKeyValueObservation

If the `NSKeyValueObservation` was being deinited on one thread while a
KVO change notice was being broadcast on another, it could end up trying
to handle the change notice concurrently with deiniting, which will
probably crash.

This also fixes a problem where it was swizzling `NSObject`'s
`observeValue(forKeyPath:of:change:context:)` implementation, which
resulted in potentially invoking undefined behavior if an observer ever
called up to `NSObject`'s implementation.

https://bugs.swift.org/browse/SR-9074
https://bugs.swift.org/browse/SR-9075

aschwaighofer/swift,aschwaighofer/swift,gribozavr/swift,ahoppen/swift,atrick/swift,harlanhaskins/swift,jckarter/swift,harlanhaskins/swift,lorentey/swift,nathawes/swift,devincoughlin/swift,karwa/swift,atrick/swift,jmgc/swift,jmgc/swift,xwu/swift,lorentey/swift,xedin/swift,allevato/swift,allevato/swift,apple/swift,apple/swift,shahmishal/swift,allevato/swift,jmgc/swift,sschiau/swift,tkremenek/swift,benlangmuir/swift,amraboelela/swift,xwu/swift,aschwaighofer/swift,shajrawi/swift,rudkx/swift,benlangmuir/swift,gregomni/swift,hooman/swift,jckarter/swift,xedin/swift,shajrawi/swift,xwu/swift,parkera/swift,rudkx/swift,JGiola/swift,parkera/swift,jmgc/swift,CodaFi/swift,tkremenek/swift,roambotics/swift,gribozavr/swift,hooman/swift,shajrawi/swift,shahmishal/swift,stephentyrone/swift,apple/swift,lorentey/swift,xedin/swift,harlanhaskins/swift,airspeedswift/swift,xwu/swift,ahoppen/swift,tkremenek/swift,practicalswift/swift,hooman/swift,amraboelela/swift,ahoppen/swift,glessard/swift,CodaFi/swift,airspeedswift/swift,jmgc/swift,benlangmuir/swift,stephentyrone/swift,harlanhaskins/swift,stephentyrone/swift,hooman/swift,gregomni/swift,parkera/swift,stephentyrone/swift,amraboelela/swift,benlangmuir/swift,devincoughlin/swift,shahmishal/swift,glessard/swift,roambotics/swift,allevato/swift,shahmishal/swift,airspeedswift/swift,shahmishal/swift,karwa/swift,hooman/swift,sschiau/swift,parkera/swift,sschiau/swift,shajrawi/swift,allevato/swift,jckarter/swift,xedin/swift,rudkx/swift,gribozavr/swift,karwa/swift,benlangmuir/swift,devincoughlin/swift,lorentey/swift,gribozavr/swift,allevato/swift,CodaFi/swift,nathawes/swift,harlanhaskins/swift,sschiau/swift,amraboelela/swift,gribozavr/swift,practicalswift/swift,airspeedswift/swift,roambotics/swift,JGiola/swift,aschwaighofer/swift,rudkx/swift,gribozavr/swift,devincoughlin/swift,lorentey/swift,tkremenek/swift,ahoppen/swift,glessard/swift,parkera/swift,parkera/swift,karwa/swift,aschwaighofer/swift,nathawes/swift,hooman/swift,glessard/swift,xedin/swift,gribozavr/swift,nathawes/swift,shajrawi/swift,devincoughlin/swift,karwa/swift,xwu/swift,shajrawi/swift,amraboelela/swift,rudkx/swift,gregomni/swift,jmgc/swift,devincoughlin/swift,xedin/swift,amraboelela/swift,gribozavr/swift,gregomni/swift,shajrawi/swift,atrick/swift,practicalswift/swift,atrick/swift,roambotics/swift,jckarter/swift,jckarter/swift,shahmishal/swift,CodaFi/swift,tkremenek/swift,nathawes/swift,sschiau/swift,roambotics/swift,allevato/swift,harlanhaskins/swift,glessard/swift,airspeedswift/swift,roambotics/swift,parkera/swift,apple/swift,lorentey/swift,sschiau/swift,practicalswift/swift,ahoppen/swift,practicalswift/swift,xedin/swift,harlanhaskins/swift,shajrawi/swift,devincoughlin/swift,JGiola/swift,nathawes/swift,karwa/swift,aschwaighofer/swift,jckarter/swift,stephentyrone/swift,JGiola/swift,practicalswift/swift,CodaFi/swift,jmgc/swift,JGiola/swift,gregomni/swift,JGiola/swift,tkremenek/swift,xwu/swift,lorentey/swift,sschiau/swift,ahoppen/swift,airspeedswift/swift,atrick/swift,apple/swift,gregomni/swift,jckarter/swift,atrick/swift,apple/swift,stephentyrone/swift,devincoughlin/swift,tkremenek/swift,hooman/swift,parkera/swift,sschiau/swift,nathawes/swift,xwu/swift,lorentey/swift,practicalswift/swift,xedin/swift,shahmishal/swift,rudkx/swift,CodaFi/swift,aschwaighofer/swift,karwa/swift,shahmishal/swift,benlangmuir/swift,karwa/swift,CodaFi/swift,airspeedswift/swift,practicalswift/swift,amraboelela/swift,stephentyrone/swift,glessard/swift