Add request caching options

[petermasking]

For optimizing full-stack apps we need to add cashing options. We can do this on both ends:

1. Browser caching by setting 'cache-control' header;
2. Server caching by storing RPC responses in the gateway (and/or node).

The caching must be configured per procedure:

• Type (browser of server)
• Maximum age (before refresh)

By default the caching is disabled.