Actions validate themselves

* DesiredLRP and Task validate their actions

[#82593712]

Signed-off-by: Eric Malm <emalm@pivotal.io>
Unmarshal null or omitted actions correctly

* use raw message pointers for Actions consistently

[#82593712]

Signed-off-by: Eric Malm <emalm@pivotal.io>
Return pointers from single-object getters

* Changes TaskByGuid and DesiredLRPByProcessGuid
* return nil instead of zero-valued struct

[#82593712]

Signed-off-by: Eric Malm <emalm@pivotal.io>