Allow large number of domain to be handled within the same function

[aureq]

At the moment it's not possible for LetsLambda to get issuance of too many certificate due to time limitation in Lambda (300 seconds). This limitation is caused by 2 main elements. First is the Route53 DNS propagation (up to 60 seconds) and the second is related to the time required to generate a private key.

[aureq]

The idea is to allow LetsLambda to call itself with a different payload. Each domain present in letslambda.yml would be issued/renewed within its own Lambda invocation hence allowing a lot more certificates to be renewed in parallel.

Notes:

• Lambda allows up to 100 concurrent invocation by default, but this safety limit may be increased if required.
• What could happen with Route53 is the same hosted zone is updated by multiple invocation at the same time?
• Lambda supports function versioning which means the main invocation should ensure the same version is called during sub-invocation. This should be done via the context object.

[aureq]

The DynamoDB table capacity should be updated accordingly to concurrent_execution to ensure no child call is stuck pending read or write capacity.

[aureq]

This code is considered alpha since it requires some load testing to further check the stability. To do:

• Code related DynamoDB has to be updated to properly handle concurrent read and write
• Support for extra long queries has to be handled (ie: very long list of domains to renew)
• Lambda invoke call must be updated to handle exceptions and throttling events