Fix intermittent crashing in requestAccessWithCompletion-handler

Thread 10 Crashed:
0   libsqlite3.dylib                     0x000000018ffaaf60 sqlite3_value_text + 3384
1   libsqlite3.dylib                     0x000000018ff9d92c sqlite3_step + 4780
2   libsqlite3.dylib                     0x000000018ff9c858 sqlite3_step + 472
3   AppSupport                           0x0000000187b9d264 CPSqliteStatementSendResults + 84
4   AppSupport                           0x0000000187ba2ee8 CPRecordStoreProcessRecordStatementWithPropertyIndices + 196
5   AppSupport                           0x0000000187ba3258 CPRecordStoreProcessQueryWithBindBlock + 136
6   AppSupport                           0x0000000187ba3314 CPRecordStoreCopyAllInstancesOfClassWhereWithBindBlock + 136
7   AddressBook                          0x0000000182bb89c8 ABCCopyArrayOfAllPeopleInSourceWithSortOrdering + 84
8   Mobilimeet                           0x000000010037b9e0 __48-[APAddressBook loadContactsOnQueue:completion:]_block_invoke (APAddressBook.m:82)
9   AddressBook                          0x0000000182c0a728 __37-[ABTCC accessRequestWithCompletion:]_block_invoke + 44
10  libdispatch.dylib                    0x0000000190244014 _dispatch_call_block_and_release + 20
11  libdispatch.dylib                    0x0000000190243fd4 _dispatch_client_callout + 12
12  libdispatch.dylib                    0x000000019024b2b8 _dispatch_root_queue_drain + 552
13  libdispatch.dylib                    0x000000019024b4fc _dispatch_worker_thread2 + 72
14  libsystem_pthread.dylib              0x00000001903d96bc _pthread_wqthread + 352
15  libsystem_pthread.dylib              0x00000001903d954c start_wqthread + 0

The completion handler is called on an arbitrary thread, but accessing the same address book instance need to be serialized to one thread, see Apple documentation reference below. In practice I've only seen crashes here when using the -loadContacts: API that is in it's turn using -loadContactsOnQueue:completion suppling the main queue as the queue argument.

So, since requestAccessWithCompletion is called on the main thread we dispatch_async to the main queue for the ABAddressBookCopyOfAllPeople call and then another dispatch_async to the queue supplied for execution of the API-consumers completionBlock.

Reading:
* https://developer.apple.com/library/ios/documentation/AddressBook/Reference/ABAddressBookRef_iPhoneOS/Reference/reference.html#//apple_ref/c/func/ABAddressBookRequestAccessWithCompletion
* http://alxsrg.com/?p=190