Support prepared statements

Now that we've [addressed this in CitusDB](https://www.citusdata.com/blog/20-sumedh/250-announcing-citusdb-41), we want to get it into `pg_shard` as well. We know how to do prepared statements for queries, but `pg_shard` must support modifications as well. Adding this support will remove a _ton_ of friction from database adapters (such as Psycopg in Python, `pg` in Ruby, or the JDBC adapters in Java).