Change default semantic to not support anytime migration

[ericjbohm]

Anytime migration is not in use by any production application. Support requires a non-trivial amount of machinery in the runtime system and creates performance overhead. Anytime migration should be an opt-in feature. Either in something like CkOpts, or a subclass of CkArray, or some other mechanism by which it is explicitly chosen by applications that need it.

[stwhite91]

Spring cleaning of chare array broadcasts is a repercussion of this that causes overhead in memory usage: https://github.com/UIUC-PPL/charm/issues/2045

[rbuch]

Note that applications that use AtSync based load balancing without +LBSyncResume or some other mechanism to ensure that all objects have migrated before resumption (to be pedantic, resumption is actually alright, the real issue is if an array broadcast is delivered before an object migrates) may break if anytime migration is disabled.