Pure-Go cowsql client

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