NOTE: This functions exists for backwards compatibility. Please use
CreateConfig() instead. NOTE: the latter uses 'on_completion',
which is a function taking an argument, wherease CreateRPC uses
'callback' which is a function without arguments.
Optional string; for backwards compatibility, must be
Optional int or float, deadline for calls in seconds.
Optional callable, a callback triggered when this rpc
completes; takes no arguments.
Optional read policy; set to EVENTUAL_CONSISTENCY to
enable eventually consistent reads (i.e. reads that may be
satisfied from an older version of the datastore in some cases).
The default read policy may have to wait until in-flight
transactions are committed.