Support parallelism in alert controller for alert creation

[lilwan]

Is this a BUG REPORT or FEATURE REQUEST?:
FEATURE

What happened:
Currently alert controller reconciles alert one at a time and it is very time consuming when comes to create large number of alerts.

What you expected to happen:
parallelism can be added to improve the performance of the controller and reduce the waiting time

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

Environment:

• alert-manager version
• Kubernetes version :

$ kubectl version -o yaml

Other debugging information (if applicable):

- controller logs:

$ kubectl logs