ios: make sure no expensive operations are run in the main thread (#185)
ios: make sure no expensive operations are run in the main thread (#185)

Defer all operations to a background thread. There is no need to fulfill
the promises in the main thread, so don't jump to it unnecessarily.

Additionally, fix checking for permissions for every API call and
simplify its management by avoiding saving state.

Last, don't lazily initialize the EKEventStore to avoid potential access
from several threads.
wmcmahan/react-native-calendar-events,wmcmahan/react-native-calendar-events,wmcmahan/react-native-calendar-events