Cancel pending transactions when calling destroy

[AndreMaz]

This PR cancels pending transactions (under the hood calls clearTimeout() ) when the destroy() method is called.

Rationale, after destroy() is called it does not make sense to wait for a response hence we need to cancel all pending requests.

How to test:

• On modbus-sever, add a delay of 10secs on getHoldingRegister()
• Set client timeout to 7 secs
• Call readHoldingRegisters() in the example below
• after 5 secs try to destroy the client

Repro example

"use strict";

const ModbusRTU = require("../index");
const client = new ModbusRTU();

async function main() {
  try {
    await client.connectTCP("127.0.0.1", { port: 8502 });
    console.log("Connected");
    client.setID(1);
    client.setTimeout(7_000);

    // Disconnect after 5 seconds
    setTimeout(async () => {
      try {
        console.log(`Destroying connection at ${new Date().toISOString()}`);
      await new Promise((resolve, reject) => {
        // Try to close the connection
        client.destroy((err) => {
          if (err) return reject(err);

          console.log(`Destroyed closed at ${new Date().toISOString()}`);
          return resolve();
        });
      });
      } catch (error) {
        console.log(`Failed to destroy connection: ${error.message} at ${new Date().toISOString()}`);
      }
      
    }, 5000);

    await readRegistersOneByOne();
  } catch (error) {
    console.log(error.message);
  } finally {
    client.close();
  }
}

const spec = {
  telemetry: {
    regA: {
      address: 1,
      registerCnt: 1,
    },
  },
};

async function readRegistersOneByOne() {
  for (const [key, { address, registerCnt }] of Object.entries(
    spec.telemetry
  )) {
    try {
      console.log(`Reading registers at ${address} with ${registerCnt} registers`);
      const data = await client.readHoldingRegisters(address, registerCnt);
      console.log(key, data.buffer);
    } catch (error) {
      console.log(error);
    }
  }
}

main();

Without "cleanup"

With "cleanup"

[yaacov]

Thank you for the pull request 💚