docs: add a new subpage for reusing TCP connections in Node.js

[wsierakowski]

Issue #, if available:
Multiple issues logged in aws-sdk-js repository from users discussing the default keepAlive behaviour:

• Reusing the connection to dynamoDB calls aws/aws-sdk-js#146
• Support KeepAlive for HTTP requests aws/aws-sdk-js#671
• Use Connection: keep-alive to improve application performance aws/aws-sdk-js#2571

Description of changes:
I would like to contribute a new page to the "Node.JS considerations" section of the documentation to inform the aws-sdk-js users of the importance of reusing TCP connections to gain substantial performance improvement for their apps using AWS services. I know of many developers, including myself, who had assumed that connections are reused by default and found out about the problem only when the application was struggling to cope with the number of inbound requests and when the high volume of connections was observed at the gateway through which the outbound traffic flows through (in our company setup).

I understand that the default behaviour can't be changed for compatibility reasons, therefore I'm proposing to address this issue by making the users aware of this nuance.

I would like to also give credits to the authors of these two resources that helped me better understand and then address this issue:

• https://theburningmonk.com/2019/02/lambda-optimization-tip-enable-http-keep-alive/
• https://medium.com/expedia-group-tech/under-the-hood-of-http-requests-in-node-69e27e2cd528

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[Doug-AWS]

Thanks Woj. I'll run this by the JS team and let you know how it goes.

[Doug-AWS]

I'm accepting this and will do some editing once it's merged.

[joebowbeer]

I see merge conflict markers in the merged reusing md file.

[Doug-AWS]

I'll fix them, thanks.