PgUpdate support

[vietj]

Support updates via prepared statements with an API similar to PgQuery:

PgPreparedStatement ps = client.prepare("INSERT INTO USERS (id, name) VALUES ($1, $2));
PgUpdate update = ps.update("julien", "Julien Viet");
update.execute(ar -> {
  // Handle result
});

// Or fluently
ps.update("julien", "Julien Viet").execute(ar -> {
  // Handle result
});