Caching global data

[vouillon]

We may want to have an option to marshal global data only once for all our clients, rather than doing the same work for each initial page. If they are served from a distinct url, then we could take advantage of caching by proxies and browsers as well.

[vasilisp]

It shouldn't be too complicated, given that we already use a separate URL for client apps. Pointers:

• https://github.com/ocsigen/eliom/blob/558c528b769a1d7f749de3b1a1674bcb524d3ebc/src/lib/eliom_registration.server.ml#L1165
• https://github.com/ocsigen/eliom/blob/558c528b769a1d7f749de3b1a1674bcb524d3ebc/src/lib/eliom_registration.server.ml#L1503
• https://github.com/ocsigen/eliom/blob/558c528b769a1d7f749de3b1a1674bcb524d3ebc/src/lib/eliom_client.client.ml#L57
• https://github.com/ocsigen/ocsigen-start/blob/3cbbd9d59fbbc2d2e7fc2b2e1c48599511b33af2/template.distillery/mobile!eliom_loader.ml#L89