Service tasks

[mtitov]

RP introduced a notion of "service" tasks, which are bound to the pilot (part of the pilot description) and not registered in the Task Manager. These tasks are pushed by RP Agent, and they are launched before any other [regular] tasks are pulled by RP Agent.

Within EnTK, such tasks are referred as global service tasks, and expected to be running during the whole run. While expanding the notion of service tasks, EnTK will support service tasks per pipeline and per stage. Thus, pipeline/stage are responsible to push such service tasks before any other [regular] tasks (e.g., within stage.pre_exec), and stop/cancel them after all [regular] tasks within pipeline/stage are done (e.g., within stage.post_exec)