Merge dynamic known fields, added new cron task

[jesusgn90]

Changes:

• The way we refresh an index pattern has been changed. Now we are fetching all the dynamic fields that we may be missing and it merges those fields with our known fields.
• Added a new cron task in our server side, it refreshes all valid index patterns every 2 minutes.
• Removed refreshing action per every parent state change.
• Keep the refreshing action for all the index patterns if kbn-vis fails due to missing fields*.
• Keep the refreshing action for all the index patterns if the app health check is fired*.
• Keep the refreshing action for the selected pattern only when using the index pattern selector*.
• Wrapped into a double try/catch block, so the server won't be crashed under any error that this feature may throw.

* Now, the refreshing action is using the merging feature described above