require('./lib/lambda-service') should not reload module every time

[elundh]

I was testing out lambda-warmer with a concurrency of 2 and I could not figure out why it was upwards of 2 seconds to run. By
adding let lambda = null; to the top of index.js and

changing line 81 to be
if(!lambda){
lambda = require('./lib/lambda-service');
}
It cuts down on the time needed to run each subsequent warming ping by a lot because it will only load the library once.
With this change I am seeing much quicker run times which will save money. I wanted to put in a PR but didn't have permission.

[elundh]

yes that would be the exact solution.

[naorpeled]

Hey!
I'm currently on vacation and can't test this but feel free to open a PR and I'll happily review when I get back :)

[elundh]

Sure! I made a pr, Thanks for making this library it's pretty much exactly what I was thinking I was going to have to make and it's a lot cheaper to keep lambda warm than provisioned concurrency.