improve setting of max connections in AWS client

things can go badly wrong if the S3A FS creates a thread pool for IO > than the number of pooled AWS http connections (set by property MAXIMUM_CONNECTIONS); you also need some for any other IO requests coming in.

The max connections property is currently independent of thread pool size, and has a default value of 1.

this is why there is a troubleshooting section in the docs showing the stack trace and instructions to fix".

Better: have a dynamic minimum like thread pool size + n, for a value of n to be chosen.