caching system when running as local transform

[cvandeplas]

When running as local transform it would be good to have a caching system to allow MISP_maltego to not query everything from the MISP instance.

The challenge in implementing this lies in the question "how long should a cache be valid?"

Usually data in MISP will likely not change within minutes, on the other hand feature request #18 From Maltego to MISP will require immediate interaction.

Ideas:

• only work if it's running as local transform. (as remote transform it would breach data confidentiality)
• implement caching by default for X minutes
• if upload to MISP is done, purge cache (completely or for the edited event)