Add setting to automatically refresh the index

Lots of nondeterministic test failures are caused by forgetting to refresh the index after writing or before reading. It would be convenient if elastomer-client had a setting which handled refreshing automatically.

For performance, it could be "smart" and only refresh if a write has happened since the last refresh.

Start by locating elastomer-client's write, read, and index-refresh entry points, then review any existing tests around index visibility. Done means a configurable automatic refresh handles the described write/read cases and avoids refreshing when no write has occurred since the last refresh.

索引モデルが issue の本文から書いたものです。