Add support for backoff rate for failed allocations.

[gkorland]

The current allocation mechanism doesn't use any backoff mechanism which causes the some of the DBs to "punish" the requester.

See: simple-backoff

[sandfox]

Whilst there is no mechaminsm for this in the pool itself, it's reasonably easy to add to the the user supplied factory function.

taken from another issue...

'use strict'

const genericPool = require('generic-pool')
const retry = require('retry-as-promised')

let createAttempts = 0

const retryLog = function(msg){
  console.log(msg)
}

const create = function () {
  createAttempts++
  if(createAttempts < 5){
    return Promise.reject(new Error(`failing on attempt ${createAttempts}`))
  }
  return Promise.resolve({name:'thingy'})
}

const factory = {
  create: function () {
     return retry(create, {backoffBase: 1000, max: 10,name: 'factory.create', report:retryLog})
  },
  destroy: function () { return Promise.resolve() }
}

const pool = genericPool.createPool(factory)

pool.on("factoryCreateError", e => {
  console.log("factoryCreateError", e);
});

pool.acquire().then((resource)=>{
  console.log('got a resource', resource)
}).catch((err)=>{
  console.log('could not get a resource')
})

For the moment this is probably the best place to add it till I come up with a good plan.

[t3hmrman]

Hey @sandfox , Is this still a feature you're considering putting in this library, or would you rather prefer leaving it as a common pattern/cookbook thing?