Currenty, the function entry is hard-coded to be 'main.main' which is
not flexible. This patch adds support for specifying module name and
function name in the code package.
When a package function is executed, a service url is cached so it's
faster when function is executed again. But this will lead to problem
if function is never executed for a long time.
This patch add an expiry machanism which will release resources in
Allow user to create function using her own docker image including
all the code and dependencies inside. The image needs to have an
script running as entry point, expecting user's input as params.
This allow users to execute functions written in any language.