Ensure popcorn cleanly handles failures

[bxatnarf]

Popcorn doesn't cleanly handle all use cases. For example, it will block the kernel from shutting down while it is waiting for a page. We need to enumerate and fix other such failures/behaviors.

[bxatnarf]

Related issues:
#58
#59
#60

[bxatnarf]

With regards to failure enumeration: we should check how popcorn fails in every combination of popcorn protocol states among multiple hosts: one acting as host where process is spawned, a host from which the process is being migrated, and a host to which a process is being migrated.

[bxatnarf]

Related issue: #64

[bxatnarf]

Question: when a multithreaded and non-migrated process invokes syscalls such as mmap and munmap or page fault handlers, do other threads block? Is it ok that all threads block when when a remote thread invokes the same kernel operations?