[JENKINS-69534] Make computer creation thread safe (#7087)

This change fixes a race condition that caused file descriptor leaks
when cloud agents were created.

The Jenkins.computers map is changed from a CopyOnWriteMap to a
ConcurrentHashMap to simplify concurrent creation of computers. This
allows us to make AbstractCIBase's createNewComputerForNode and
updateNewComputer methods thread safe and simplify removeComputer.