Add sink/source callback hooks for "Connection reset by peer" exceptions

The `layer` branch is `okio-parent-0.9.0`, which is the version of okio used by the `layer` branch on our fork of okhttp.

This PR includes optional hooks that call out to a supplied IOExceptionObserver whenever the socket experiences an exception during read or write ops.  This is intended for use with the forthcoming PR in okhttp.

NOTE: This builds a custom `0.9.0-LAYER` version of this artifact, which we'll reference in our custom version of okhttp.