Changed Executor::resources to option type.

Yet another split out of the task-info RR (r19795/r18403) which turns
Executor::resources into an option type. It is needed when the
executor info isn't known up-front (the main motivation behind the
task-info RR) and the executor resources therefore cannot be known
until after the containerizer has launched the container.

Review: https://reviews.apache.org/r/20219
Removed unused using process::UPID clause in src/cli/resolve.cpp.

Review: https://reviews.apache.org/r/20310