Support multiple actors on the same rank (#494)

Summary:

Previously, debug sessions were keyed by rank, but this doesn't allow for multiple actors on the same rank. This diff updates the debugging logic so that it is keyed on actor name + rank instead of just rank. The `attach` and `cast` commands now require the user to specify actor name in addition to rank. The `list` command now sorts breakpoints by grouping all actors with the same name (i.e., from the same actor mesh) together, and then sorts by rank within each group.

Differential Revision: D78047960

Author: Sam Lurye