Avoid flat-cloning connection options per command

from https://github.com/mariadb-corporation/mariadb-connector-nodejs/pull/353

Each command merges its options over the connection options with a flat clone:

this.opts = Object.assign({}, connOpts, cmdParam.opts);

connOpts is a 63-key V8 dictionary-mode object, and flat-cloning it costs far more than the key count implies (around16 µs/command locally). Using a prototype-chain merge instead:

this.opts = Object.assign(Object.create(connOpts), cmdParam.opts);

skips the copy entirely and leaves a fast-mode object behind.

(possible because the merged object is only ever read by name, and never enumerated/spread/serialized).

this doesn't affect "string form" command, like

await connection.query('SELECT 1');

on command with "object form" like :

await connection.query({sql: 'SELECT 1', rowsAsArray:true});

Still this are what most frameworks uses, so worth this ticket.

Impact is around 1.75× throughput on small commands over local TCP, e.g. query(

): change from 38 µs to 21 µs/command.