v2writable: done: force synchronous awaitpid

v2writable: done: force synchronous awaitpid

We need to shut down shards synchronously to reliably release
the inbox write lock when inside the DS event loop (as the
lei/store subprocess is, unlike most v2writable users).

This seems to fix long-running `lei import' failures to
lei/store after repeated tests.  It is a good idea anyways to
ensure exit status of shard workers are correct before returning
from ->done.