Add CamelAsyncClosure.

Identical to EAsyncClosure in libedataserver, for use in Camel.
CamelService: Use CamelAsyncClosure.

Relinquish its internal 'AsyncClosure' struct, which is identical.
camel_store_get_inbox_folder_sync: Remove lock.

Don't see any reason to lock here.  Providers ought to be able to handle
this concurrently with other methods.  Let any side-effects reveal them-
selves so we can fix them or document them.
camel_subscribable_folder_is_subscribed: Remove lock.

Don't see any reason to lock here.  Providers ought to be able to handle
this concurrently with other methods.  Let any side-effects reveal them-
selves so we can fix them or document them.