Fix apply and _generate_key.

Removes the 'key_' prefix from keys made with _generate_key

Removes the ability for users to create their own names for variables on
the engines. Instead a uid with the '__distarray__' prefix is generated.

Otherwise, if users can create arbitrailily named variables, we won't
know what strings represent proxy objects on the engines. Since user
named variables won't have the '__distarray__' prefix.
Fix tests to reflect changes to context.apply API.
Modify @kwsmith's regression test to reflect new API