fix(pg-cursor): settle every read when reads overlap

[Kesavamoorthig06]

Problem

When two read() calls on the same Cursor overlap, the first promise never settles.

Repro (against any running postgres):

const { Client } = require('pg')
const Cursor = require('pg-cursor')

const client = new Client({ connectionString: process.env.DATABASE_URL })
await client.connect()
const cursor = client.query(new Cursor('select 1 union all select 2'))

const a = cursor.read(1)
const b = cursor.read(1)
await b          // resolves: [ { "?column?": 1 } ]
await a          // hangs forever

Cause

The second read() submits while the connection is still busy with the first read's portal. handleRowDescription (and _ifNoData) unconditionally shift the submission queue and overwrite this._cb while this._state === 'busy', clobbering the first read's callback. On top of that, _sendRows returns after delivering rows without shifting the queue, so a read that was queued behind an in-flight read is never started when the connection goes idle.

Fix

• In handleRowDescription / _ifNoData, only shift the queue and install _cb when the connection is not busy; a busy connection means the next read's portal was already sent by next and the shift happened there.
• In _sendRows, after delivering rows, if the connection is idle and more reads are queued, start the next one via _shiftQueue.

Adds a regression test (concurrent reads settle in order) that fails without the fix (mocha 2s timeout) and passes with it.