Pure-Go dqlite client

 Go-dqlite is a pure-Go dqlite client implementation. 2 binaries are
 provided:
   - dqlite-demo, a demo dqlite application, which exposes a simple
     key/value store over an HTTP API,
   - dqlite, a basic SQLite-like dqlite shell which supports
     normal SQL queries plus the sp ecial .cluster and .leader
     commands to inspect the cluster members and the current leader.