connection string determines length of connection

[shortthefomo]

##Motivation

🚨 WARNING! 🚨 If you are using xrpl-client in a serverless environment, make sure to .close() your connection at the end, to prevent your connection to linger on for a long time (causing massive load on public infrastructure, and massive serverless function invocation time bills from your cloud provider!)

unless a none cluster node is provided in the connection string the lib should auto kill the connection after a period of inactivity or possibly a simple timeout.

this will help move some on to their own infrastructure.

[WietseWind]

@lathanbritz I personally feel the client should do what it is supposed to do: keep a connection.

The infrastructure it connects to should be in charge of closing connections, imo?